Abstract:
In a heating device, a detector detects AC voltage of the AC power source. A control device detects a zero-cross timing of the AC voltage by using the detector and generate a signal having a high level period of time, a low level period of time, a rising transition changing from a low level to a high level, and a falling transition changing from the high level to the low level. The signal is such a form that at least one of the rising transition and the falling transition is in incoincidence with the zero-cross timing. The control device controls the heating switch to perform a switching action in which the heating switch is rendered on when the zero-cross timing is detected during the high level period of time and rendered off when the zero-cross timing is detected during the low level period of time.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from Japanese Patent Application No. 2013-135991 filed Jun. 28, 2013. The entire content of the priority application is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to a technology for controlling the supply of an AC voltage to a heating body. 
       BACKGROUND 
       [0003]    Conventionally, components of image-forming devices have been driven by AC power supplied from a commercial power supply. Japanese Patent Application Publication No. 2012-208450 describes a technology for controlling electricity conducted to a heater by switching a conduction signal at a timing based on the detection timing for zero-cross points in the outputted AC voltage. With this type of image-forming device, it is desirable to synchronize the timing at which the conduction signal is switched with the timing at which a zero-cross point is detected in order to reduce noise and power consumption. 
       SUMMARY 
       [0004]    In order to synchronize the switch timing of the conduction signal with the detection timing of a zero-cross point, it is necessary to toggle the conduction signal at the timing that a zero-cross point is detected. To accomplish this, the conventional image-forming device generates an internal signal separate from the conduction signal and switches this internal signal together with the conduction signal. In this way, the image-forming device can determine, at the detection timing of a zero-cross point, that the conduction signal is being switched to ON if the internal signal is on and that the conduction signal is being switched to OFF if the internal signal is off. 
         [0005]    According to this method, the internal signal must be switched at a relatively short period corresponding to the timing at which the zero-cross points are detected. However, if the timing at which the internal signal is switched overlaps the timing at which a zero-cross point is detected, the state of the internal signal may be indeterminate when the image-forming device is switching the conduction signal, leading to instability in the control of electricity conducted to the heater or other heating body. 
         [0006]    In view of the foregoing, it is an object of the present invention to provide a technology for controlling the conduction of electricity to a heating body based on zero-cross points of an AC voltage. 
         [0007]    In order to attain the above and other objects, the invention provides a heating device may include a heating body, a heating switch, a detector, and a control device. The heating body may be configured to be connected to an AC power source and generate heat by power supplied from the AC power source. The heating switch may be configured to be connected between the AC power source and the heating body and configured to switch supply of the power from the AC power source to the heating body. The detector may be configured to detect AC voltage of the AC power source. The control device may be configured to: detect a zero-cross timing of the AC voltage by using the detector; generate a signal having a high level period of time, a low level period of time, a rising transition changing from a low level to a high level, and a falling transition changing from the high level to the low level, the signal being such a form that at least one of the rising transition and the falling transition is in incoincidence with the zero-cross timing; and control the heating switch to perform a switching action in which the heating switch is rendered on when the zero-cross timing is detected during the high level period of time and rendered off when the zero-cross timing is detected during the low level period of time. 
         [0008]    According to another aspect, the present invention provides an image forming device. The image forming device may include an image forming unit, and a fixing device. The image forming unit may be configured to print an image on a sheet. The fixing device may be configured to fix the image on the sheet. The fixing device may include a heating body, a heating switch, a detector, and a control device. The heating body may be configured to be connected to an AC power source and generate heat by power supplied from the AC power source. The heating switch may be configured to be connected between the AC power source and the heating body and configured to switch supply of the power from the AC power source to the heating body. The detector may be configured to detect AC voltage of the AC power source. The control device may be configured to: detect a zero-cross timing of the AC voltage by using the detector; generate a signal having a high level period of time, a low level period of time, a rising transition changing from a low level to a high level, and a falling transition changing from the high level to the low level, the signal being such a form that at least one of the rising transition and the falling transition is in incoincidence with the zero-cross timing; and control the heating switch to perform a switching action in which the heating switch is rendered on when the zero-cross timing is detected during the high level period of time and rendered off when the zero-cross timing is detected during the low level period of time. 
         [0009]    According to still another aspect, the present invention provides a method for controlling a heating device. The heating device includes: a heating body configured to be connected to an AC power source and generate heat by power supplied from the AC power source; a heating switch configured to be connected between the AC power source and the heating body and configured to switch supply of the power from the AC power source to the heating body; and a detector configured to detect AC voltage of the AC power source. The method includes: detecting a zero-cross timing of the AC voltage by using the detector; generating a signal having a high level period of time, a low level period of time, a rising transition changing from a low level to a high level, and a falling transition changing from the high level to the low level, the signal being such a form that at least one of the rising transition and the falling transition is in incoincidence with the zero-cross timing; and controlling the heating switch to perform a switching action in which the heating switch is rendered on when the zero-cross timing is detected during the high level period of time and rendered off when the zero-cross timing is detected during the low level period of time. 
         [0010]    According to still another aspect, the present invention provides a non-transitory computer readable storage medium storing a set of program instructions installed on and executed by a computer for controlling a heating device including: a heating body configured to be connected to an AC power source and generate heat by power supplied from the AC power source; a heating switch configured to be connected between the AC power source and the heating body and configured to switch supply of the power from the AC power source to the heating body; and a detector configured to detect AC voltage of the AC power source. The program instructions includes: detecting a zero-cross timing of the AC voltage by using the detector; generating a signal having a high level period of time, a low level period of time, a rising transition changing from a low level to a high level, and a falling transition changing from the high level to the low level, the signal being such a form that at least one of the rising transition and the falling transition is in incoincidence with the zero-cross timing; and controlling the heating switch to perform a switching action in which the heating switch is rendered on when the zero-cross timing is detected during the high level period of time and rendered off when the zero-cross timing is detected during the low level period of time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
           [0012]      FIG. 1  is a cross-section of an printer according to a first embodiment of the invention; 
           [0013]      FIG. 2  is a block diagram illustrating a voltage supply circuit according to the first embodiment; 
           [0014]      FIG. 3  is a circuit diagram illustrating a power cutoff switch according to the first embodiment; 
           [0015]      FIG. 4  is a circuit diagram illustrating a zero-cross detection circuit according to the first embodiment; 
           [0016]      FIG. 5  is a flowchart illustrating an image forming process according to the first embodiment; 
           [0017]      FIG. 6  is a timing chart illustrating an internal signal and a zero-cross signal according to the first embodiment; 
           [0018]      FIG. 7  is a timing chart illustrating a maximum fixing duration and a period number, and a cumulative number according to the first embodiment; 
           [0019]      FIG. 8  is a circuit diagram illustrating a zero-cross detection circuit according to a second embodiment; 
           [0020]      FIG. 9  is a flowchart illustrating an image forming process according to the second embodiment; 
           [0021]      FIG. 10  is a block diagram illustrating a voltage supply circuit according to a third embodiment; 
           [0022]      FIG. 11  is a flowchart illustrating an image forming process according to the third embodiment; and 
           [0023]      FIG. 12  is a timing chart illustrating an internal signal and a zero-cross signal according to the third embodiment. 
       
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
       [0024]      FIG. 1  shows a printer  10  according to an first embodiment. The printer  10  in the embodiment is a direct transfer laser printer that functions to form images, and is an example of the image-forming device according to the present invention. 
         [0025]    The printer  10  includes a casing  12  for accommodating the components of the printer  10 . Within the casing  12 , the printer  10  includes a paper tray  14 , a pressing plate  18 , a pickup roller  20 , conveying rollers  22 , registration rollers  24 , an image-transferring unit  30 , and an image-forming unit  40 . The paper tray  14  is disposed in the bottom section of the casing  12  and accommodates stacked sheets  16  of paper or the like. The user mounts the paper tray  14  in the casing  12  after loading the paper tray  14  with sheets  16 . The pressing plate  18  is disposed in the paper tray  14  for pressing the sheets  16  upward on one end so that the topmost sheet  16  is pressed against the pickup roller  20 . The pickup roller  20  rotates to convey the topmost sheet  16  to the conveying rollers  22 , and the conveying rollers  22  convey the sheet  16  to the registration rollers  24 . After correcting skew in the sheet  16 , the registration rollers  24  convey the sheet  16  to the image-transferring unit  30  and the image-forming unit  40 . Together, the image-transferring unit  30  and the image-forming unit  40  are an example of the image-forming unit. 
         [0026]    The printer  10  is also provided with a first sensor  25  downstream of the registration rollers  24 . The first sensor  25  detects whether a sheet  16  is being conveyed toward the image-transferring unit  30  and the image-forming unit  40 . The first sensor  25  is on when a sheet  16  is being conveyed toward the image-transferring unit  30  and the image-forming unit  40  and off when a sheet  16  is not being conveyed. The first sensor  25  is an example of the sensors according to the invention. 
         [0027]    The image-transferring unit  30  includes a pair of support rollers  32  and  34 , a belt  36 , and a transfer roller  37 . The belt  36  has a loop shape and is mounted around the support rollers  32  and  34 . The transfer roller  37  is disposed inside the loop formed by the belt  36 . The support rollers  32  and  34  are rotated counterclockwise in  FIG. 1  by a motor (not shown), and the belt  36  circulates along with this rotation. 
         [0028]    The image-forming unit  40  is disposed above the belt  36 . The image-forming unit  40  includes a scanning unit  42 , and a process unit  44 . The scanning unit  42  is disposed above a photosensitive drum  48  (described later) of the process unit  44 . A central processing unit (see  FIG. 2 ; hereinafter “CPU”)  62  described later controls the scanning unit  42  to irradiate a laser beam L over the surface of the photosensitive drum  48  based on image data transferred from a memory unit  64  (described later; see  FIG. 2 ) that is configured of RAM, ROM, or the like. 
         [0029]    The process unit  44  includes the photosensitive drum  48 , and a developer cartridge  46 . The developer cartridge  46  is filled with toner. In an image-forming operation, the scanning unit  42  irradiates the laser beam L over the surface of the photosensitive drum  48  to form an electrostatic latent image corresponding to the image being printed. Toner in the developer cartridge  46  is then supplied to the latent image to form a toner image on the surface of the photosensitive drum  48 . 
         [0030]    As the toner image formed on the surface of the photosensitive drum  48  rotates through a transfer position P 1  between the photosensitive drum  48  and the belt  36 , the toner image is transferred from the photosensitive drum  48  onto a sheet  16  passing through the transfer position P 1 . In this way, an image is formed on the sheet  16 . Subsequently, the belt  36  conveys the sheet  16  to a fixing unit  52  described below. 
         [0031]    The printer  10  further includes a second sensor  27 , and a registration sensor  29 . The second sensor  27  is disposed upstream of the fixing unit  52  and detects the presence of a sheet  16  being conveyed to the fixing unit  52 . The second sensor  27  is on when a sheet  16  is being conveyed to the fixing unit  52  and off when a sheet  16  is not being conveyed to the fixing unit  52 . The registration sensor  29  is disposed in a position for confronting the belt  36  at the support roller  34  and functions to detect toner deposited on the belt  36 . 
         [0032]    The fixing unit  52  includes a fixing heater  54 , fixing rollers  28 , and a temperature gauge  57 . The fixing heater  54  generates heat when an AC voltage from an AC power supply  50  (see  FIG. 2 ) is supplied to the fixing heater  54  via a voltage supply circuit  56 . Heat generated by the fixing heater  54  thermally fixes the transferred image to the sheet  16 . The temperature gauge  57  detects the temperature of the fixing heater  54 . The fixing heater  54  is an example of the heating body and, in combination with the voltage supply circuit  56 , is an example of the heating device. 
         [0033]    Subsequently, pairs of conveying rollers  26  disposed downstream of the fixing unit  52  convey the sheet  16  out of the casing  12  and onto a discharge tray  38  provided on the top surface of the casing  12 . In this way, the pickup roller  20 , the conveying rollers  22 , and other various rollers constitute a conveying unit  58  that serves to convey sheets  16  along a conveying path  47  that leads from the paper tray  14  to the image-transferring unit  30 , the image-forming unit  40 , and the fixing unit  52 . 
         [0034]    As shown in  FIG. 2 , the voltage supply circuit  56  includes a control circuit  60 , a power cutoff switch  72 , a zero-cross detection circuit  74 , and a switching power supply circuit  78 . 
         [0035]    The zero-cross detection circuit  74  and the switching power supply circuit  78  in the voltage supply circuit  56  are connected in parallel to the AC power supply  50 . The fixing heater  54  is also connected in parallel to these circuits through the power cutoff switch  72 . 
         [0036]    The switching power supply circuit  78  converts an AC voltage supplied from the AC power supply  50  to DC voltage to be supplied to the control circuit  60  and the like. The control circuit  60  includes an application-specific integrated circuit (ASIC)  66 , and a memory unit  64 . In addition to the CPU  62 , the ASIC  66  includes an internal clock generation circuit  63  and other dedicated hardware circuits. The memory unit  64  stores various programs for controlling the operations of the printer  10 . The CPU  62  reads programs from the memory unit  64  and controls the components of the printer  10  according to the programs. More specifically, the CPU  62  generates an internal signal NS (see  FIG. 6 ) based on an internal clock CL (see  FIG. 6 ) generated by the internal clock generation circuit  63  and executes an image-forming process described later. The control circuit  60  is an example of the control device. 
         [0037]    The power cutoff switch  72  is provided on a power supply line DL via which the AC voltage of the AC power supply  50  is outputted to the fixing heater  54 . The power cutoff switch  72  switches the supply of the AC voltage to the fixing heater  54  on or off based on a signal inputted from the CPU  62 . More specifically, the power cutoff switch  72  is a phototriac coupler. As shown in  FIG. 3 , the power cutoff switch  72  includes a photodiode  72 A, and a phototriac element  72 B. The photodiode  72 A is a light-emitting element that is connected to the control circuit  60 . The phototriac element  72 B is a light-receiving element that is connected to the fixing heater  54 . The power supply line DL is an example of the conducting path. The power cutoff switch  72  is an example of the heating switch. 
         [0038]    The power cutoff switch  72  is known as a zero-cross type that operates in synchronization with a zero-cross timing ZT of a zero-cross signal ZS (see  FIG. 6 ) detected by the zero-cross detection circuit  74  and inputted into the power cutoff switch  72  via the CPU  62 . More specifically, the photodiode  72 A emits light based on the internal signal NS generated by the CPU  62 , and the power cutoff switch  72  switches to an ON state for supplying AC voltage to the fixing heater  54  in synchronization with the zero-cross timing ZT when the zero-cross signal ZS arrives at the zero-cross timing ZT while the power cutoff switch  72  is emitting light. The power cutoff switch  72  switches to an OFF state for interrupting the supply of AC voltage to the fixing heater  54  in synchronization with the zero-cross timing ZT when the zero-cross signal ZS arrives at a zero-cross timing ZT while the photodiode  72 A is not emitting light. 
         [0039]    Returning to  FIG. 2 , the zero-cross detection circuit  74  detects the AC voltage outputted along the power supply line DL and generates a zero-cross signal ZS having an ON voltage while the absolute value of the AC voltage is greater than a reference value KV 1  (see  FIG. 6 ) and an OFF voltage while the absolute value is less than or equal to the reference value KV 1 . The zero-cross detection circuit  74  is an example of the detector. 
         [0040]    As shown in  FIG. 4 , the zero-cross detection circuit  74  includes a rectifier circuit W 1 , a photo-coupler PC 1 , resistors R 1 -R 3 , and an inverter circuit H 1 . The rectifier circuit W 1  is connected to the AC power supply  50  and is a diode bridge that performs full-wave rectification of the AC voltage. The AC voltage converted by the rectifier circuit W 1  is outputted to the photo-coupler PC 1 . 
         [0041]    The photo-coupler PC 1  includes a photodiode D 1 , and a phototransistor Q 1 . When the value of the AC voltage after undergoing full-wave rectification by the rectifier circuit W 1  is greater than the reference value KV 1 , electric current flows to the photodiode D 1 , causing the photodiode D 1  to emit light and the phototransistor Q 1  to turn on. On the other hand, when the rectified voltage is less than or equal to the reference value KV 1 , electric current does not flow to the photodiode D 1 , turning the phototransistor Q 1  off. Hence, the phototransistor Q 1  is on while the value of the rectified AC voltage is greater than the reference value KV 1 , and off when less than or equal to the reference value KV 1 . 
         [0042]    When the phototransistor Q 1  of the photo-coupler PC 1  turns on, current flows through the resistors R 1 -R 3  and the phototransistor Q 1 , causing the voltage value at a terminal TN 1  provided between the resistors R 1  and R 2  to drop from an ON voltage to an OFF voltage. The CPU  62  detects the voltage at the terminal TN 1  via the inverter circuit H 1 . When the voltage value at the terminal TN 1  drops from an ON voltage to an OFF voltage, the value of the voltage outputted by the inverter circuit H 1  rises from an OFF voltage to an ON voltage. On the other hand, when the phototransistor Q 1  turns off, current does not flow through the resistors R 1 -R 3  and the phototransistor Q 1 , causing the voltage value at the terminal TN 1  to rise from an OFF voltage to an ON voltage. Consequently, the voltage outputted from the inverter circuit H 1  drops from an ON voltage to an OFF voltage. This configuration enables the CPU  62  to detect the zero-cross signal ZS. 
         [0043]    When detecting the zero-cross signal ZS, the CPU  62  determines a low voltage duration of time TL (see  FIG. 6 ) in which the voltage value is equivalent to the OFF voltage. The CPU  62  finds a center point within the low voltage duration of time TL of the zero-cross signal ZS and determines that this timing (center point) is the zero-cross timing ZT of the AC voltage. 
         [0044]    Next, an image-forming process for forming an image on a sheet  16  will be described with reference to  FIGS. 5 through 7 . When the user turns on the power supply of the printer  10 , the CPU  62  begins executing an image-forming process at prescribed intervals that have been predetermined. During this process, the CPU  62  executes a control process for controlling the supply of electricity to the fixing heater  54 . 
         [0045]    In S 2  at the beginning of the image-forming process shown in  FIG. 5 , the CPU  62  detects the period (cycle) of the AC voltage supplied from the AC power supply  50 . To determine the period of the AC voltage, the CPU  62  detects the zero-cross timing ZT using the zero-cross detection circuit  74  and multiplies the period T 0  of the zero-cross timing ZT by 2. The period T 0  is an example of the zero-cross-to-zero-cross period of time. 
         [0046]    In S 4  the CPU  62  sets a reference period T 2  for setting a pulse pattern of the internal signal NS. The internal signal NS is a binary signal that is switched between an ON voltage for controlling the photodiode  72 A of the power cutoff switch  72  to emit light, and an OFF voltage for controlling the photodiode  72 A not to emit light. The CPU  62  determines the reference period T 2  to be used in setting the pulse pattern of the internal signal NS, i.e., the rise time at which the internal signal NS is at the ON voltage and the fall time at which the internal signal NS is at the OFF voltage. The reference period T 2  is an example of the reference period. 
         [0047]    Specifically, the CPU  62  receives data from the internal clock generation circuit  63  indicating the period T 1  of the internal clock CL and compares the period T 1  of the internal clock CL to the period T 0  of the zero-cross timing ZT. The CPU  62  establishes an integer N such that a multiple N of the period T 1  is not an integer multiple of the period T 0 , and uses the multiple N of the period T 1  as the reference period T 2 . Accordingly, the reference period T 2  is set to a different interval from an integer multiple of the period T 0  for the zero-cross timing ZT. Thus, the end of the interval for the reference period T 2  is at a different timing than a zero-cross timing ZT when the start of the interval is synchronized with a zero-cross timing ZT, as shown in  FIG. 6 . 
         [0048]    In the example of the embodiment shown in  FIG. 6 , the reference period T 2  is set to an interval longer than the period T 0  of the zero-cross timing ZT and shorter than the period of the AC voltage. As will be described later, the high level period of the internal signal NS can be set to three times the reference period T 2  in the embodiment when rapidly heating the fixing heater  54 . Further, the low level period of the internal signal NS can be set to three times the reference period T 2  when slowly heating the fixing heater  54 , such as when the temperature of the fixing heater  54  is near the target temperature. For this reason, the reference period T 2  in the embodiment is set such that three times the reference period T 2  is a different interval from an integer multiple of the period T 0  for the zero-cross timing ZT. 
         [0049]    In addition, the reference period T 2  in the embodiment is set based on the maximum size of a sheet  16  on which the image-forming unit  40  can form images. A maximum sheet size and a maximum fixing duration ST are preset on the printer  10 . The maximum sheet size is the largest sheet  16  on which the image-forming unit  40  can form an image, such as an A3-size sheet, as shown in  FIG. 7 . The maximum fixing duration ST is the longest duration of time that the internal signal NS must be outputted in order for the fixing unit  52  to fix an image on a sheet  16  of the maximum size, based on the conveying speed of the conveying unit  58 . If a time difference ΔT is found by subtracting the period T 0  of the zero-cross timing ZT from the reference period T 2  (see Equation 1) and a period number KN is found by dividing the maximum fixing duration ST by the reference period T 2  to indicate the number of reference periods T 2  included in the maximum fixing duration ST (see Equation 2), then in the embodiment a cumulative time ET equivalent to the product of the time difference ΔT and period number KN is set to be shorter than the period T 0  of the zero-cross timing ZT (see Equation 3). 
         [0000]      Δ T=T 2− T 0   Equation 1
 
         [0000]        KN=ST/T 2   Equation 2
 
         [0000]        T 0&gt;Σ T=ΔT×KN    Equation 3
 
         [0050]    After setting the reference period T 2  for the internal signal NS, in S 6  the CPU  62  determines whether the user has inputted a print command. If a print command has not been inputted (S 6 : NO), the CPU  62  ends the current image-forming process. 
         [0051]    However, if the CPU  62  determines that the user has inputted a print command (S 6 : YES), in S 8  the CPU  62  controls the internal clock generation circuit  63  to begin outputting the internal clock CL and starts an internal timer in the CPU  62 . In S 10  the CPU  62  controls the conveying unit  58  to begin conveying the sheet  16 . In S 11  the CPU  62  determines whether the leading edge of the sheet  16  has passed the first sensor  25  based on output from the first sensor  25 . While the leading edge of the sheet  16  has not passed the first sensor  25  (S 11 : NO), the CPU  62  again detects the period of the AC voltage in S 20  similarly to S 2  and sets the reference period T 2  in S 22  similarly to S 4 . 
         [0052]    The CPU  62  may execute a measurement process prior to conveying the sheet  16  for a printing process if the condition for executing the measurement process was satisfied before the print command was inputted, for example. In the measurement process, the CPU  62  controls the image-transferring unit  30  to form a test pattern on the belt  36  and controls the registration sensor  29  to measure the test pattern. Since the sheet  16  is not being conveyed while the measurement process is being executed, the CPU  62  determines in S 11  that the leading edge of the sheet  16  has not passed the first sensor  25  (S 11 : NO). In such a case, the CPU  62  detects the period of the AC voltage in S 20  and resets (or reconfigures) the reference period T 2  in S 22  because the AC voltage may change while the measurement process is being executed. 
         [0053]    When the CPU  62  determines that the trailing edge of the sheet  16  has passed the first sensor  25  (S 11 : YES), in S 12  the CPU  62  controls the image-transferring unit  30  to form a toner image on the sheet  16  when the sheet  16  is conveyed to the transfer position Pl. 
         [0054]    In S 14  the CPU  62  detects the temperature of the fixing heater  54  using the temperature gauge  57  and compares the detected temperature with the fixing temperature required for the fixing heater  54  to fix an image on the sheet  16 . In S 16  the CPU  62  generates an internal signal NS pattern based on the results of comparing the detected temperature to the fixing temperature. 
         [0055]    When the difference calculated by subtracting the detected temperature from the fixing temperature is greater than a first reference temperature difference, the CPU  62  sets the pulse pattern of the internal signal NS to a pattern that is repeated every four reference periods T 2  and includes an ON signal for three continuous reference periods T 2  followed by an OFF signal for one reference period T 2 . In other words, the CPU  62  generates an internal signal NS pattern in which the high level period is three times the reference period T 2  and the low level period is the reference period T 2 . Since the high level period is longer than the low level period in this internal signal NS pattern, the time during which AC voltage is supplied to the fixing heater  54  can be relatively long and the fixing heater  54  can be heated rapidly, as will be described later. 
         [0056]    However, if the difference calculated by subtracting the detected temperature from the fixing temperature is less than or equal to the first reference temperature difference and greater than a second reference temperature difference that is smaller than the first reference temperature difference, the CPU  62  sets the pulse pattern of the internal signal NS to a pattern with a signal that alternates on and off for each reference period T 2  as shown in  FIG. 6 . In other words, the CPU  62  generates an internal signal NS pattern in which both the high level period and the low level period are the reference period T 2 . 
         [0057]    Additionally, if the difference calculated by subtracting the detected temperature from the fixing temperature is less than or equal to the second reference temperature difference, the CPU  62  sets the pulse pattern of the internal signal NS to a pattern that is repeated every four reference periods T 2  and includes an ON signal for one reference period T 2  followed by an OFF signal for three continuous reference periods T 2 . In other words, the CPU  62  generates an internal signal NS pattern in which the high level period is the reference period T 2  and the low level period is three times the reference period T 2 . Since the low level period is longer than the high level period in this internal signal NS pattern, the time for supplying an AC voltage to the fixing heater  54  can be relatively short so that the fixing heater  54  can be heated slowly, as will be described later. 
         [0058]    In S 24  the CPU  62  begins outputting the internal signal NS. As described above, when the zero-cross signal ZS reaches the zero-cross timing ZT during a high level period of the internal signal NS, the power cutoff switch  72  either begins to supply an AC voltage to the fixing heater  54  in synchronization with the zero-cross timing ZT or maintains the current status of the AC voltage supply. When the zero-cross signal ZS arrives at the zero-cross timing ZT during a low level period of the internal signal NS, the power cutoff switch  72  either interrupts the supply of the AC voltage to the fixing heater  54  in synchronization with the zero-cross timing ZT or maintains the interrupted status of the AC voltage supply. In other words, the CPU  62  uses wavenumber control to control the fixing heater  54  based on the AC voltage. 
         [0059]    As shown in  FIG. 6 , in the embodiment, the initial rise timing of the internal signal NS is set to be synchronized with the zero-cross timing ZT. Because the high level period and low level period of the internal signal NS are generated as integer multiple of the reference period T 2 . Here, the reference period T 2  is defined so as to satisfy the equations 1-3. Accordingly, except the initial rise timing of the internal signal NS, the rise timing and fall timing of the internal signal NS are not coincide with the zero-cross timing ZT at least in a maximum fixing duration ST starting from the initial rise timing of the internal signal NS. 
         [0060]    After initiating output of the internal signal NS, in S 26  the CPU  62  determines whether the trailing edge of the sheet  16  has passed over the second sensor  27  based on output from the second sensor  27 . Hence, after the second sensor  27  turns on, the CPU  62  waits while the second sensor  27  remains on (S 26 : NO). When the second sensor  27  turns off (S 26 : YES), in S 28  the CPU  62  halts output of the internal signal NS to interrupt the AC voltage supply to the fixing heater  54 . 
         [0061]    In S 30  the CPU  62  determines whether the image-forming process has been completed. If the CPU  62  determines based on the print command that the image-forming process has not been completed, such as when the inputted print command specifies a plurality of sheets  16  but only one sheet  16  has undergone image formation (S 30 : NO), then the CPU  62  returns to S 11  and prepares to form an image on the next sheet  16 . 
         [0062]    When a plurality of sheets  16  is conveyed in succession, the CPU  62  provides a prescribed gap between each pair of consecutively conveyed sheets  16  to ensure that the sheets  16  do not overlap. Since the first sensor  25  is off during the gaps between consecutively conveyed sheets  16 , the CPU  62  can use this interval in which the first sensor  25  is off to execute the processes in S 20  and S 22 . If a measurement condition is satisfied during an image-forming operation, the CPU  62  may execute a measurement process after completing image formation on one sheet  16  and prior to conveying the next sheet  16 , for example. The CPU  62  executes the processes in S 20  and S 22  while this measurement process is being executed. 
         [0063]    When the CPU  62  determines that the image-forming process has been completed for the sheets  16  indicated in the print command (S 30 : YES), in S 36  the CPU  62  controls the image-transferring unit  30  to halt the formation of toner images and conveyance of the sheet, ending the current image-forming process. 
         [0064]    (1) In the printer  10  according to the embodiment, the power cutoff switch  72  is a zero-cross type phototriac coupler that is used to switch the supply of AC voltage to the fixing heater  54  on and off. The power cutoff switch  72  switches the AC voltage supply on or off based on the status of the internal signal NS at the zero-cross timing ZT. In the embodiment, at least one of the rise timing and fall timing of the internal signal NS is set different from the zero-cross timings ZT. Accordingly, the status of the internal signal NS at a zero-cross timing ZT can be clearly discerned, reducing the likelihood of any variations in the results of determining whether or not to supply AC Voltage to the fixing heater  54 . Thus, this configuration can suppress instability in temperature control for the fixing heater  54 . 
         [0065]    (2) More specifically, the reference period T 2  used to set the high level period and low level period of the internal signal NS are set to an interval different from the period T 0  of the zero-cross timings ZT. Therefore, when the rise timing of the internal signal NS is synchronized with the zero-cross timing ZT, for example, the fall timing of the internal signal NS will differ from the zero-cross timing ZT and, in most cases, both the rise timings and fall timings will come at different timings than the zero-cross timing ZT. 
         [0066]    (3) In the printer  10  of the embodiment, the reference period T 2  is set longer than the period T 0  of the zero-cross timings ZT. Since the printer  10  performs temperature control to raise the temperature of the fixing heater  54  to a fixing temperature, it is preferable to use a method of control that can increase the temperature more rapidly. By setting the reference period T 2  longer than the period T 0  of the zero-cross timings ZT, the printer  10  can more rapidly increase the temperature of the fixing heater  54  than when the reference period T 2  is set shorter than the period T 0  of the zero-cross timings ZT. 
         [0067]    (4) In the printer  10  of the embodiment, the reference period T 2  is set based on the maximum size of a sheet  16  on which the image-forming unit  40  can form images. Specifically, if the cumulative time ET is the accumulation of time differences AT, denoting the difference between the reference period T 2  and the period T 0  of the zero-cross timing ZT, over the maximum fixing duration ST for a maximum size sheet  16 , the reference period T 2  is set such that the cumulative time ET is shorter than the period T 0  of the zero-cross timing ZT. This arrangement keeps the rise timing or fall timing of the internal signal NS from coinciding with a zero-cross timing ZT less than or equal to one time while an image is being fixed on the sheet  16 , thereby suppressing instability in temperature control for the fixing heater  54 . 
         [0068]    When the initial rise timing of the internal signal NS is synchronized with the zero-cross timing ZT as in the example of  FIG. 6 , temperature control of the fixing heater  54  can be particularly unstable at the starting point of heating the fixing heater  54  prior to fixing an image on the sheet  16 . Such instability can produce blemishes on the sheet  16  when occurring during the fixing process. However, after the process for fixing an image on the sheet  16  has begun, the rise timings and fall timings of the internal signal NS will not coincide with the zero-cross timing ZT. Accordingly, the configuration of the embodiment reduces instability in temperature control for the fixing heater  54  while an image is being fixed on the sheet  16 . 
         [0069]    (5) If the supply of AC voltage to the fixing heater  54  is interrupted during the conveying period for conveying the sheets  16  (i.e., between the start and end of the conveying period), the printer  10  of the embodiment regenerates (or reconfigures) the internal signal NS. By updating the internal signal NS in response to changes in environmental conditions within the device, such as temperature and humidity, the printer  10  can better control the temperature of the fixing heater  54  with consideration for its environment than when continuing to use the same internal signal NS set at the start of the image-forming process. 
       Second Embodiment 
       [0070]    Next, a second embodiment of the present invention will be described with reference to  FIGS. 8 and 9 . The second embodiment differs from the first embodiment in that the zero-cross detection circuit  74  has a switch  74 A for toggling on and off the AC voltage supply to the zero-cross detection circuit  74 . The following description will focus on points of difference from the first embodiment, wherein like parts and components are designated with the same reference numerals to avoid duplicating description. 
         [0071]    As shown in  FIG. 8 , the zero-cross detection circuit  74  according to the second embodiment has a structure similar to the zero-cross detection circuit  74  according to the first embodiment, but also includes the switch  74 A. The switch  74 A is an example of the detecting switch. 
         [0072]    The switch  74 A includes a photo-coupler PC 2 , a resistor R 4 , and a transistor TR 1 . The photo-coupler PC 2  further includes a photodiode D 2 , and a phototransistor Q 2 . The photodiode D 2  is connected to the transistor TR 1  via the resistor R 4 . The phototransistor Q 2  is provided on a closed circuit along which AC voltage is supplied after full rectification by the rectifier circuit W 1 . The rectified AC voltage is supplied to the photodiode D 1  of the photo-coupler PC 1  via the phototransistor Q 2 . 
         [0073]    The CPU  62  outputs a control signal SS to the transistor TR 1 . When the transistor TR 1  is turned on by the control signal SS, electric current flows to the photodiode D 2 , causing the photodiode D 2  to emit light and the phototransistor Q 2  to turn on. On the other hand, when the transistor TR 1  is turned off by the control signal SS outputted from the CPU  62 , the electric current does not flow to the photodiode D 2 , turning the phototransistor Q 2  off. 
         [0074]    When the phototransistor Q 2  of the photo-coupler PC 2  is on, the AC voltage fully rectified by the rectifier circuit W 1  is supplied to the photo-coupler PC 1  through the phototransistor Q 2 , enabling the CPU  62  to detect the zero-cross signal ZS. On the other hand, when the phototransistor Q 2  is off, the voltage supply to the photo-coupler PC 1  is interrupted, and the CPU  62  is unable to detect the zero-cross signal ZS. Hence, the switch  74 A functions to switch on and off the supply of AC voltage to the zero-cross detection circuit  74  based on the control signal SS outputted from the CPU  62 . 
         [0075]    In S 42  at the beginning of the image-forming process shown in  FIG. 9 , the CPU  62  begins outputting the control signal SS to the transistor TR 1  of the switch  74 A shown in  FIG. 8 . Initially, the control signal SS is an ON voltage for turning on the transistor TR 1 . With this configuration, when an AC voltage is first supplied to the zero-cross detection circuit  74 , in S 2  the CPU  62  detects the period of the AC voltage by detecting the zero-cross timing ZT using the zero-cross detection circuit  74 . In S 4  the CPU  62  sets the reference period T 2  of the internal signal NS based on the period of the detected AC voltage. 
         [0076]    After detecting the period of the AC voltage and setting the reference period T 2  for the internal signal NS, in S 44  the CPU  62  enters a standby state to stand ready to detect the period of the AC voltage, and halts output of the control signal SS to the transistor TR 1 . That is, the CPU  62  outputs an OFF voltage to the transistor TR 1  of the switch  74 A for turning off the transistor TR 1 . Turning off the transistor TR 1  interrupts the supply of AC voltage to the zero-cross detection circuit  74 . 
         [0077]    In other words, the CPU  62  begins outputting the control signal SS to the transistor TR 1  prior to detecting the period of the AC voltage and stops outputting the control signal SS to the transistor TR 1  after the period has been detected. The CPU  62  repeatedly switches the output status of the control signal SS each time the period of the AC voltage is detected. 
         [0078]    The printer  10  according to the second embodiment described above interrupts the AC voltage supply to the zero-cross detection circuit  74  during a standby state in which the CPU  62  waits to detect the period of the AC voltage. With this configuration, the printer  10  can reduce power consumption in the zero-cross detection circuit  74  during the standby state. 
       Third Embodiment 
       [0079]    Next, a third embodiment of the present invention will be described with reference to  FIGS. 10 through 12 . The third embodiment differs from the first embodiment in that the pattern of the internal signal is generated based on a peak detection signal PS. The following description will focus on points of difference from the first embodiment, wherein like parts and components are designated with the same reference numerals to avoid duplicating description. 
         [0080]    As shown in  FIG. 10 , the voltage supply circuit  56  according to the third embodiment has a similar configuration to the voltage supply circuit  56  according to the first embodiment, but also includes a peak-detecting circuit  76 . The peak-detecting circuit  76  in the voltage supply circuit  56  and the fixing heater  54  via the power cutoff switch  72  are connected in parallel to the AC power supply  50 . 
         [0081]    The peak-detecting circuit  76  detects the AC voltage outputted on the power supply line DL and generates a peak detection signal PS (see  FIG. 12 ). The peak detection signal PS includes an OFF voltage in intervals where the absolute value of the detected AC voltage is greater than a reference value KV 2  set greater than the reference value KV 1  (see  FIG. 12 ) and an ON voltage in intervals where the absolute value of the detected AC voltage is less than or equal to the reference value KV 2 . 
         [0082]    While not diagramed in  FIG. 10 , the peak-detecting circuit  76  is configured similarly to the zero-cross detection circuit  74  shown in  FIG. 4 , excluding the inverter circuit H 1 . In addition, the resistors R 1 -R 3  connected in series to the phototransistor Q 1  in the peak-detecting circuit  76  have different resistance values than in the zero-cross detection circuit  74 . As a result, the peak detection signal PS detected by the CPU  62  is an OFF voltage when the value of the fully-rectified AC voltage is greater than the reference value KV 2 , which is larger than the reference value KV 1 , and is an ON voltage when the value of the fully-rectified AC voltage is less than or equal to the reference value KV 2 . By detecting the peak detection signal PS, the CPU  62  detects the low voltage duration of time TL in which the voltage value is equivalent to the OFF voltage. The CPU  62  finds a center point within the low voltage duration of time TL of the peak detection signal PS and determines that this timing is a peak timing PT of the AC voltage. 
         [0083]    In S 52  at the beginning of the image-forming process shown in  FIG. 11 , the CPU  62  detects the zero-cross signal ZS using the zero-cross detection circuit  74  and determines the zero-cross timing ZT of the zero-cross signal ZS. In S 54  the CPU  62  sets the reference period T 2  of the internal signal NS. Specifically, the CPU  62  detects the peak timing PT using the peak-detecting circuit  76  and sets the reference period T 2  to the period T 0  of the peak timing PT. 
         [0084]    If the CPU  62  determines in S 6  that the user has inputted a print command (S 6 : YES), then in S 56  the CPU  62  generates an internal signal NS pattern. The CPU  62  generates this pattern based on the peak timings PT. Specifically, the CPU  62  generates an internal signal NS pattern in which the high level period and low level period of the internal signal NS are integer multiples of the period T 0  and the rise timing and fall timing of these periods coincides with a peak timing PT. Subsequently, in S 24 -S 28  the CPU  62  controls the AC voltage supply to the fixing heater  54  based on the internal signal NS having the pattern established above. In S 58  the CPU  62  performs the same process with S 56 . 
         [0085]    The printer  10  according to the third embodiment sets the rise timing and fall timing of the internal signal NS shifted from the zero-cross timings by one-fourth the period of the AC voltage, i.e., half the period of the zero-cross timings ZT. With this configuration, the printer  10  can clearly discern the status of the internal signal NS at each zero-cross timing ZT to suppress any variation in results of determining whether to supply AC voltage to the fixing heater  54 . Accordingly, the printer  10  can suppress instability in temperature control for the fixing heater  54 . 
         [0086]    While the invention has been described in detail with reference to the embodiments thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention. 
         [0087]    (1) In the embodiments, the device having a printing function (that is, the printer  10 ) is explained as an example of the present invention. However, the present invention is applicable to other devices such as a multifunction peripheral having a scanner function and facsimile function in addition to the printing function. The present invention can be applied to a wide variety of devices that control the supply of electricity to heating bodies, such as the fixing heater  54 , during processes. 
         [0088]    (2) In the embodiments, the control device  60  includes the ASIC  66  and the ASIC  66  includes the CPU  62 . The CPU  62  executes the image forming process or other processes by using hardware included in the ASIC  66  if needed. However, the control device  60  may include a CPU different from the ASIC  66 , and this CPU executes each process. Alternatively, the ASIC  66  may not include the CPU  62  and hardware included in the ASIC h 66  may execute each process. Further, one CPU, a plurality of CPUs, one ASIC, or a plurality of ASICs may be included in the control device  60  and execute each process. 
         [0089]    (3) The programs executed by the CPU  62  need not be stored in the memory unit  64 , but may be stored in the ASIC  66  or another storage device. 
         [0090]    (4) The first embodiment describes an example of setting the reference period T 2  for the internal signal NS based on the maximum size of a sheet  16  on which the image-forming unit  40  can form images, but the present invention is not limited to this configuration. For example, if a particular size of sheet  16 , such as the A4-size sheet, is imagined to be the most frequently used size when forming images, the reference period T 2  of the internal signal NS may be determined based on the size of this sheet  16 . Further, it is not necessary for the reference period T 2  to be set based on the size of a sheet  16 . 
         [0091]    (5) In the example of the first embodiment, the reference period T 2  is set to an interval longer than the period T 0  of the zero-cross timings ZT, but the reference period T 2  may be set to a shorter interval than the period T 0  instead. 
         [0092]    (6) The reference period T 2  may also be set equivalent to the period T 0  of the zero-cross timings ZT, as in the third embodiment. It is not necessary that the reference period T 2  be set to an interval different from the period T 0 , provided that the rise timing and fall timing of the internal signal NS can be set to a different timing than the zero-cross timings ZT, such as a timing between the peak timings PT and the zero-cross timings ZT. One example of this timing is an intermediate timing between the peak timings PT and the zero-cross timings ZT. 
         [0093]    (7) In the examples of the second and third embodiments, the internal signal NS is not regenerated during the image-forming process, but the internal signal NS may be regenerated (that is, the processes S 20  and S 22  are executed) during the image-forming process as described in the first embodiment.