Patent Publication Number: US-2006018676-A1

Title: Image forming apparatus and control method therefor

Description:
FIELD OF THE INVENTION  
      The present invention relates to an image forming apparatus and its control method and, more particularly, to safety control on a plurality of fixing devices in an image forming apparatus having the fixing devices.  
     BACKGROUND OF THE INVENTION  
      Conventionally, in the market of copying machines and printers which are required to cope with various kinds of paper and be highly productive, in order to meet these requirements, an arrangement having a plurality of fixing devices in one image forming apparatus has been proposed. When, for example, a toner image transferred on a paper sheet is heated and pressurized by a fixing device in order to fix the image on a paper sheet, there is a need to arbitrarily adjust the glossiness of a printed image. In order to satisfy this need, a fixing apparatus has been proposed (see, for example, Japanese Patent Laid-Open No. 2000-221821), which has a plurality of fixing devices  1 ,  2 , and  3  arranged in series in the paper passing direction. These fixing devices arbitrarily adjust the glossiness of an image to be printed by switching the numbers of nips formed in the respective fixing devices and the positions where nips are used, when a sheet on which an unfixed toner image is formed is made to pass through the nips in the fixing devices to fix the unfixed toner image on the sheet.  
      On the other hand, a fixing device in an image forming apparatus generally reaches a high temperature, and thus high safety precausions for users are required. For this reason, when the temperature of the fixing heater of a fixing device exceeds the upper limit value of a temperature control range or does not reach the lower limit value of the temperature control range, processing for such an abnormality must be performed by ON/OFF-controlling the input of an AC power supply. This technique is generally introduced into currently available products.  
      For example, a target temperature is set for a fixing device in accordance with the state of the printer, and an upper and lower limit temperatures are set for each target temperature. When the temperature of the temperature-controlled fixing device exceeds the upper limit temperature or does not reach the lower limit temperature, it is determined that the device has failed. In addition, there has been proposed a method in which when the temperature of the fixing device has exceeded the above upper limit temperature or has not reached the above lower limit temperature, different failure determination processes are performed depending on whether this has happened after the start of paper feed or after the reception of a vertical sync signal (VSYNC) (see, for example, Japanese Patent Laid-Open No. 5-11663).  
      For the arrangement above which has a plurality of fixing devices arranged in series in the paper passing direction and switches the numbers of nips formed in the respective fixing devices and the positions where nips are used (Japanese Patent Laid-Open No. 2000-221821), there is conceivable an arrangement in which the abnormality processing method for a fixing device which is disclosed in Japanese Patent Laid-Open No. 5-11663 is applied to each fixing device and allows each fixing device to perform this abnormality processing independently. When it is determined that a fixing device is abnormal, in an image forming apparatus having such an arrangement, and the fixing heater of the fixing device in which the abnormality has found is turned off, it appears as if safety on the image forming apparatus was maintained.  
      In an image forming apparatus such as that described above, which has a plurality of fixing devices arranged in series, however, when one fixing device becomes abnormal and its fixing heater is turned off, there is a possibility that the ambient temperature is still high. In this case, the operation of peripheral circuits cannot be guaranteed, and for example, the temperature of an adjacent fixing device cannot be properly detected. As a consequence, a secondary abnormality may occur.  
      In addition, it is not preferable in terms of safety that while an abnormality in one fixing device is detected and no image cannot be formed, power is kept supplied to other fixing devices in which no abnormality has been detected. In addition, such operation is wasteful in terms of power consumption.  
     SUMMARY OF THE INVENTION  
      The present invention has been initially made to solve the above problems in the prior arts, and has as its object to provide an image forming apparatus and its control method in which even if an abnormality occurs in at least one of a plurality of fixing devices in which the image forming apparatus has, image forming operation can be conducted more safely by stopping the supply of power to all the fixing devices.  
      In order to achieve the above object, an image forming apparatus according to an aspect of the present invention has the following arrangement. The arrangement includes a plurality of fixing devices each including a fixing heater which fixes a developing agent on a recording medium, a switch which turns on/off supply of power to the fixing heater, and an abnormality detection unit which detects an abnormality of the fixing heater, and a control unit which turns off all the switches of the plurality of fixing devices so as to stop supply of power to all the plurality of fixing devices, when an abnormality is detected by the abnormality detection unit of at least one of the fixing devices, regardless of a detection result by the abnormality detection unit of the other fixing devices.  
      In order to achieve the above object, an image forming method for an image forming apparatus according to another aspect of the present invention has the following steps. There is provided a control method for an image forming apparatus including a plurality of fixing devices each including a fixing heater which fixes a developing agent on a recording medium, a switch which turns on/off supply of power to the fixing heater, and an abnormality detection unit which detects an abnormality of the fixing heater, comprising an abnormality detection step of detecting an abnormality of the fixing heater by using the abnormality detection unit, and a control step of turning off all the switches of the plurality of fixing devices so as to stop supply of power to all the plurality of fixing devices, when an abnormality is detected by the abnormality detection unit of at least one of the fixing devices, regardless of a detection result by the abnormality detection unit of the other fixing device.  
      In order to achieve the above object, an image forming apparatus according to still another aspect of the present invention has the following arrangement. The arrangement includes a plurality of fixing devices each including a fixing heater which fixes a developing agent on a recording medium, a switch which turns on/off supply of power to the fixing heater, and a serial communication unit which performs communication by using serial data, and a control unit which turns off all the switches of the plurality of fixing devices to stop supply of power to all the plurality of fixing devices when a communication abnormality is detected in the serial communication unit of at least one of the fixing devices.  
      According to the image forming apparatus and its control method of the present invention, in an image forming apparatus having a plurality of fixing devices, when the occurrence of an abnormality in at least one of the fixing devices is detected, the supply of power to all the fixing devices can be stopped. This makes it possible to form an image more safely.  
      Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
       FIG. 1  is a block diagram showing a control arrangement (software control) for an image forming apparatus having a plurality of fixing devices according to an embodiment of the present invention;  
       FIG. 2  is a flowchart showing processing to be performed when an abnormality in a fixing device is detected in the control arrangement shown in  FIG. 1 ;  
       FIG. 3  is a schematic view showing the internal structure of an image forming apparatus according to an embodiment of the present invention;  
       FIG. 4  is a block diagram showing a control arrangement (hardware control) for an image forming apparatus having a plurality of fixing devices according to an embodiment of the present invention;  
       FIG. 5  is a block diagram showing a control apparatus (software control) for an image forming apparatus having a plurality of fixing devices (using thermal switches) according to another embodiment of the present invention;  
       FIG. 6  is a flowchart showing processing to be performed when an abnormality in a fixing device is detected in the control arrangement shown in  FIG. 5 ;  
       FIG. 7  is a block diagram showing a control arrangement (software control) for an image forming apparatus having a plurality of fixing devices (including serial communication) according to still another embodiment of the present invention;  
       FIG. 8  is a flowchart showing processing to be performed when an abnormality in a fixing device is detected in the control arrangement shown in  FIG. 7 ; and  
       FIG. 9  is a view for explaining an example of temperature transition when a temperature abnormality in a fixing device is detected. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings.  
      A preferred embodiment of the present invention will be described in detail below with reference to the accompanying drawings.  
      As an image forming apparatus having a plurality of fixing devices according to an embodiment of the present invention, one of various types of image forming apparatuses can be used, in which an electrophotographic means, thermosensible means, thermal transfer means, ink-jet means, and the like form images sent as electrical signals on various kinds of recording media such as plain paper and thermosensible paper by using various kinds of recording agents such as toner and ink.  
      As an example of an image forming apparatus, an image forming apparatus having an image forming unit based on the electrophotographic scheme which has two fixing devices arranged in series and uses a laser beam exposure scheme will be described below. This apparatus uses a printing sheet as a recording medium and toner as a recording agent. However, the number of fixing devices is not limited to two, and three or more fixing devices may be arranged in series. In addition, as an image forming unit, a unit using a scheme other than the laser beam exposure scheme may be used, and a recording medium (e.g., various kinds of printing sheets and OHP (overhead projector) paper) and a recording agent (e.g., ink) may be selected and used in accordance with the scheme.  
      [Internal Structure of Image Forming Apparatus:  FIG. 3 ] 
       FIG. 3  is a schematic view showing the internal structure of an image forming apparatus using the laser beam exposure scheme. Referring to  FIG. 3 , reference numeral  301  denotes a photoconductive drum;  302 , a charging device (charging roller);  303 , a laser optical system;  304 , a developing device;  305 , a transfer roller,  306 - 1  and  306 - 2 , fixing devices;  307 , a paper receiving tray;  308 , a blade;  309 , an exhaust toner container;  310 , a manual feed tray;  311 , a feed roller; and  312 , a paper feed cassette. An optical semiconductor layer whose electrical characteristics change upon application of light is formed on the photoconductive drum  301 . This drum rotates at a constant speed during image forming operation. Image forming operation in the above arrangement is performed as follows: 
      (1) charging step: causing the charging device (charging roller)  302  to uniformly charge the optical semiconductor layer on the photoconductive drum  301 ;     (2) laser exposure step: causing the laser optical system  303  to apply an image pattern onto the photoconductive drum  301  to form an electrostatic latent image;     (3) development step: causing the developing device  304  to make toner adhere to the electrostatic latent image; 
        (4) transfer step: causing the transfer roller  305  to transfer the image, to which the toner adheres, onto a printing sheet;    
        (5) fixing step: causing the two fixing devices  306 - 1  and  306 - 2  arranged in series to fix the toner of the transferred image on the printing sheet by heating and pressurizing the printing sheet, and delivering the printing sheet onto the paper receiving tray  307 ; and     (6) cleaning step: causing the blade  308  to scrape off toner which is left on the photoconductive drum  301  without being transferred onto the printing sheet, and storing the scraped toner in the exhaust toner container  309 .    
      Image forming operation is performed in accordance with the above steps. Note that printing sheets are stacked on the paper feed cassette  312  or manual feed tray  310  and are conveyed to the surface of the photoconductive drum  301  by the feed roller  311  as indicated by a broken line portion A in  FIG. 3 .  
      [Control Arrangement (First Arrangement) for Executing Abnormality Processing for Fixing Device:  FIG. 1 ] 
      Assume that when image printing operation is performed by using the above image forming apparatus, at least one of the two fixing devices becomes abnormal. A control arrangement for abnormality processing in such a case will be described next.  
      For the sake of simplicity, assume that in the following description, an upper limit temperature Tmax (e.g., 230° C.) and a lower limit temperature Tmin (e.g., 70° C.) are set for the fixing heater of each fixing device, and each fixing device is considered abnormal when (1) the measured temperature of each fixing heater has exceeded the upper limit temperature Tmax for a predetermined time ta (e.g., 2 sec) or more (in case of detection of an excessive temperature rise), or (2) the measured temperature of each fixing heater has not reached the lower limit temperature Tmin (in case of detection of a low temperature) even after a lapse of a predetermined time tb (e.g., 3.5 min).  
       FIG. 1  is a block diagram showing a control arrangement (first arrangement) for performing abnormality processing for each fixing device described above. This arrangement comprises a fixing controller  101  and two fixing devices  100 - 1  and  100 - 2 . The fixing devices  100 - 1  and  100 - 2  respectively comprise relays  102 - 1  and  102 - 2 , fixing heaters  103 - 1  and  103 - 2 , temperature sensors  104 - 1  and  104 - 2 , and comparators  105 - 1  and  105 - 2 . The fixing devices  100 - 1  and  100 - 2  are respectively connected to AC power supplies  106 - 1  and  106 - 2  as external power supplies.  
      The fixing controller  1010 N/OFF-controls the two fixing heaters  103 - 1  and  103 - 2  in accordance with information such as the type of printing sheet, the temperatures of the fixing heaters, and fixing error detection information, and can control the temperatures of the fixing devices  100 - 1  and  100 - 2  or turn off the fixing heaters of the fixing devices  100 - 1  and  100 - 2  upon occurrence of an abnormality by ON/OFF-controlling the fixing heaters  103 - 1  and  103 - 2 . The relays  102 - 1  and  102 - 2  are controlled by heater ON/OFF signals  107 - 1  and  107 - 2 , and control the supply of power from the AC power supplies  106 - 1  and  106 - 2  to the fixing heaters  103 - 1  and  103 - 2 . The fixing heaters  103 - 1  and  103 - 2  generate heat for fixing toner on a printing sheet. The temperature sensors  104 - 1  and  104 - 2  measure the temperatures of the fixing heaters  103 - 1  and  103 - 2 .  
      The comparators  105 - 1  and  105 - 2  compare the temperatures measured by the temperature sensors  104 - 1  and  104 - 2  (the temperatures of the fixing heaters  103 - 1  and  103 - 2 ) with the preset temperature Tmax (predetermined temperature), and determine whether the measured temperatures exceed the predetermined temperature. If the measured temperatures exceed the predetermined temperature Tmax, output signals from the comparators  105 - 1  and  105 - 2  are activated to output error signals  1  and  2  indicated by  108 - 1  and  108 - 2  (signals indicating that the fixing heaters  103 - 1  and  103 - 2  are abnormal). If the measured temperatures do not exceed the predetermined temperature Tmax, the comparators  105 - 1  and  105 - 2  do not activate their output signals, and hence do not output error signals  1  and  2 .  
      The AC power supplies  106 - 1  and  106 - 2  serve as power supplies for the fixing heaters  103 - 1  and  103 - 2 . The signals  107 - 1  and  107 - 2  are ON/OFF signals for the relays  102 - 1  and  102 - 2 . These signals serve to control the supply of power to the fixing heaters  103 - 1  and  103 - 2 . Reference numerals  109 - 1  and  109 - 2  denote temperature signals, which are analog data representing the temperatures of the fixing heaters  103 - 1  and  103 - 2 .  
      [Processing to be Performed by First Arrangement Upon Occurrence of Abnormality in Fixing Devices:  FIG. 2 ] 
      A control flow to be executed in the above arrangement comprising the fixing devices  100 - 1  and  100 - 2  and the fixing controller  101  when the fixing controller  101  detects an abnormality in at least one of the comparators  105 - 1  and  105 - 2  of the fixing devices  100 - 1  and  100 - 2  (when an excessive temperature rise or a low temperature is detected) will be described with reference to  FIG. 2 . For the sake of descriptive convenience, the following description is made by taking the temperature transition shown in  FIG. 9  as an example of processing in accordance with the flowchart of  FIG. 2 . The following description can be applied to a temperature transition other than that shown in  FIG. 9 .  
       FIG. 9  shows an example of each of the temperature profiles of the fixing heaters  103 - 1  and  103 - 2  which are measured by the temperature sensors  104 - 1  and  104 - 2 . More specifically, each profile shown in  FIG. 9  is an example of each of the temperature profiles obtained when the fixing heaters  103 - 1  and  103 - 2  are heated while the set lower limit temperature Tmin for the fixing heaters  103 - 1  and  103 - 2  is set to 70° C., and the set upper limit temperature Tmax for the heaters is set to 230° C. The first temperature profile indicates a case of detection of an excessive temperature rise (i.e., a case wherein a temperature exceeding the set upper limit temperature Tmax (230° C.) is detected for the set time ta (2 sec) or more. The second temperature profile indicates a case of detection of a low temperature (i.e., a case wherein a temperature equal to or lower than the set lower limit temperature Tmin (70° C.) is detected for the set time tb (3.5 min) or more). The third temperature profile indicates a case wherein the heaters are normally heated (i.e., the set temperature range (70 to 230° C.)).  
      If it is determined in step S 101  in  FIG. 2  that an output signal from the comparator  105 - 1  of the fixing device  100 - 1  is activated, and the error signal  108 - 1  continues for a predetermined time (ta: 2 sec) or more (the predetermined upper limit temperature Tmax “230° C.” continues for 2 sec or more), the flow advances to step S 102  to turn off the relays  102 - 1  and  102 - 2  to stop heating the fixing heaters (i.e., the fixing heaters  103 - 1  and  103 - 2 ) of all the fixing devices (this operation corresponds to processing at time t 2  when the fixing heater  103 - 1  is heated in accordance with the first temperature profile in the case shown in  FIG. 9 ).  
      If it is determined in step S 101  that an output signal from the comparator  105 - 1  of the fixing device  100 - 1  is not active (the error signal  108 - 1  is not output), the flow advances to step S 103  to check whether the temperature signal  109 - 1  from the fixing device  100 - 1  indicates a predetermined temperature (tmin: 70° C.) or higher within a predetermined time (tb: 3.5 min). If this signal does not indicate the predetermined temperature (tmin: 70° C.) or higher within the predetermined time (tb: 3.5 min), the flow advances to step S 102  to stop heating all the fixing heaters (i.e., the fixing heaters  103 - 1  and  103 - 2 ) of all the fixing devices by turning off the relays  102 - 1  and  102 - 2  (this corresponds to the processing at time t 3  in the case shown in  FIG. 9  when the fixing heater  103 - 1  is heated in accordance with the second temperature profile).  
      If it is determined in step S 103  that the temperature signal  109 - 1  from the fixing device  100 - 1  indicates a predetermined temperature (tmin: 70° C.) or higher within a predetermined time (tb: 3.5 min), the flow advances to step S 104  (this corresponds to the processing at time t 3  in the case shown in  FIG. 9  when the fixing heater  103 - 1  is heated in accordance with the third temperature profile).  
      If it is determined in step S 104  that an output signal from the comparator  105 - 2  of the fixing device  100 - 2  is active and the error signal  108 - 2  continues for a predetermined time (ta: 2 sec) or more (when the predetermined upper limit temperature Tmax “230° C.” continues for 2 sec or more), the flow advances to step S 102  to stop heating the fixing heaters (i.e., the fixing heaters  103 - 1  and  103 - 2 ) of all the fixing devices by turning off the relays  102 - 1  and  102 - 2  (this corresponds to the processing at time t 2  in the case shown in  FIG. 9  when the fixing heater  103 - 2  is heated in accordance with the first temperature profile).  
      If it is determined in step S 104  that an output signal from the comparator  105 - 2  of the fixing device  100 - 2  is not active (the error signal  108 - 2  is not output), the flow advances to step S 105  to check whether the temperature signal  109 - 2  from the fixing device  100 - 2  indicates a predetermined temperature (tmin: 70° C.) or higher within a predetermined time (tb: 3.5 min). If the signal does not indicate the predetermined temperature (tmin: 70° C.) or higher within the predetermined time (tb: 3.5 min), the flow advances to step S 102  to stop heating the fixing heaters (i.e., the fixing heaters  103 - 1  and  103 - 2 ) of all the fixing devices by turning off the relays  102 - 1  and  102 - 2  (this corresponds to the processing at time t 3  in the case shown in  FIG. 9  when the fixing heater  103 - 2  is heated in accordance with the second temperature profile).  
      If it is determined in step S 105  that the temperature signal  109 - 2  from the fixing device  100 - 2  indicates a predetermined temperature (tmin: 70° C.) or higher within a predetermined time (tb: 3.5 min), the flow advances to step S 101  (this corresponds to the processing at time t 3  in the case shown in  FIG. 9  when the fixing heater  103 - 2  is heated in accordance with the third temperature profile).  
      As described above, the image forming apparatus of the present invention includes two fixing devices and a fixing controller. Each fixing device includes a switch which supplies power to the fixing heater, a temperature sensor which measures the temperature of the fixing heater, and a comparator which determines from the measured temperature whether or not the fixing heater is operating normally, and outputs an error signal when determining that the fixing heater is abnormal. When, therefore, at least one of the comparators outputs an abnormal signal, the fixing controller can control the switches to stop supplying power to all the fixing devices. This allows the image forming apparatus of the present invention to form an image more safely.  
      [Control Arrangement (Second Arrangement) for Performing Abnormality Processing for Fixing Devices:  FIG. 4 ] 
      The above description has exemplified the abnormality processing for the fixing devices in the arrangement comprising the fixing controller and the fixing devices each having the temperature sensor in  FIG. 1 . However, processing similar to that described above can also be performed in the arrangement shown in  FIG. 4 . Note that since the processing in  FIG. 4  is similar to the processing described with reference to  FIGS. 1 and 2 , a description of common parts will be omitted, and only points different from those in the arrangement shown in  FIG. 1  will be described below.  
       FIG. 4  is a block diagram showing a control system for a case wherein abnormality processing for each fixing device, which is described with reference  FIG. 1 , is performed by another arrangement. The control system comprises a fixing controller  201  and two fixing devices  200 - 1  and  200 - 2 . The fixing devices  200 - 1  and  200 - 2  in  FIG. 4  respectively comprise relays  202 - 1  and  202 - 2 , fixing heaters  203 - 1  and  203 - 2 , temperature sensors  204 - 1  and  204 - 2 , comparators  205 - 1  and  205 - 2 , and AND circuits  210 - 1  and  210 - 2 . The fixing devices  200 - 1  and  200 - 2  are respectively connected to AC power supplies  206 - 1  and  206 - 2  as external power supplies.  
      The arrangement shown in  FIG. 4  differs from that shown in  FIG. 1  in that when output signals from the comparators  205 - 1  and  205 - 2  are activated, an error signal  208  is not output to the fixing controller  201  unlike in  FIG. 1 , but is input to the AND circuits  210 - 1  and  210 - 2  in the fixing devices  200 - 1  and  200 - 2 , and the relays  202 - 1  and  202 - 2  are controlled by using output signals from the AND circuits  210 - 1  and  210 - 2  (in this case, the signals output from the comparators  205 - 1  and  205 - 2  are open collector outputs).  
      If the temperature of a fixing heater exceeds the set upper limit temperature, output signals from the comparators  205 - 1  and  205 - 2  are activated to output an error signal. This signal is input to the AND circuits  210 - 1  and  210 - 2 . As a consequence, the AND circuits  210 - 1  and  210 - 2  output signals to turn off the relays  202 - 1  and  202 - 2 , thereby stopping heating the fixing heaters (i.e., the fixing heaters  203 - 1  and  203 - 2 ) of all the fixing devices.  
      In this manner, in the second arrangement shown in  FIG. 4 , when the measured temperature of one of the fixing heaters of the fixing devices  200 - 1  and  200 - 2  exceeds the set upper limit temperature Tmax, it is determined that the fixing heater is abnormal, and the switches are controlled to stop the supply of power to all the fixing devices. Therefore, the image forming apparatus of the present invention can form an image more safely.  
      [Control Arrangement (Third Arrangement) for Performing Abnormality Processing for Fixing Devices:  FIG. 5 ] 
      The description of the abnormality processing for the fixing devices, which has been made with reference to  FIGS. 1 and 4 , has exemplified the arrangement in which the fixing devices use the temperature sensors. However, the present invention can be realized by an arrangement using thermal switches in place of the temperature sensors as shown in  FIG. 5 . Abnormality processing in this arrangement will be described below. A description of portions common to those in  FIGS. 1 and 2  will be omitted, and only different points will be described.  
       FIG. 5  is a block diagram showing a control system for performing abnormality processing for fixing devices in an image forming apparatus with an arrangement using thermal switches. The control system comprises a fixing controller  401  and two fixing devices  400 - 1  and  400 - 2 . The fixing devices  400 - 1  and  400 - 2  respectively comprise relays  402 - 1  and  402 - 2 , fixing heaters  403 - 1  and  403 - 2 , thermal switches  404 - 1  and  404 - 2 , temperature sensors  407 - 1  and  407 - 2 , and AND circuits  409 - 1  and  409 - 2 . The fixing devices  400 - 1  and  400 - 2  are respectively connected to AC power supplies  406 - 1  and  406 - 2  as external power supplies.  
      The fixing controller  4010 N/OFF-controls the two fixing heaters  403 - 1  and  403 - 2  in accordance with information such as the type of printing sheet, the temperatures of the fixing heaters, and fixing error detection information. The fixing controller  401  can control the temperatures of the fixing devices  400 - 1  and  400 - 2  by ON/OFF-controlling the fixing heaters  403 - 1  and  403 - 2 , and turn off the fixing heaters upon occurrence of an abnormality in the fixing devices  400 - 1  and  400 - 2 . The relays  402 - 1  and  402 - 2  are controlled by output signals from the AND circuits  409 - 1  and  409 - 2  to control the supply of power from the AC power supplies  406 - 1  and  406 - 2  to the fixing heaters  403 - 1  and  403 - 2 .  
      In this case, each of the output signals from the AND circuits  409 - 1  and  409 - 2  is controlled by three signals, i.e., one of heater ON/OFF signals  405 - 1  and  405 - 2  and signals SW 1  and SW 2  output from the thermal switches  404 - 1  and  404 - 2 . More specifically, when all the three signals are input to each of the AND circuits  409 - 1  and  409 - 2 , each AND circuit outputs an output signal. As a result, the relays  402 - 1  and  402 - 2  are operated to supply power from the AC power supplies  406 - 1  and  406 - 2  to the fixing heaters  403 - 1  and  403 - 2 . If, one of the three signals ceases to be input, for example, one of the signals SW 1  and SW 2  ceases to be input when one of the thermal switches  404 - 1  and  404 - 2  is turned off, the AND circuits  409 - 1  and  409 - 2  stop outputting output signals. As a consequence, the relays  402 - 1  and  402 - 2  are inactivated to stop the supply of power from the AC power supplies  406 - 1  and  406 - 2  to the fixing heaters  403 - 1  and  403 - 2 .  
      The fixing heaters  403 - 1  and  403 - 2  generate heat for fixing toner on a printing sheet. The thermal switches  404 - 1  and  404 - 2  are switches which turn off the circuits when the temperatures of the fixing heaters  403 - 1  and  403 - 2  become the predetermined temperature Tmax (e.g., 230° C.) or higher. The relay ON/OFF signals  405 - 1  and  405 - 2  are signals which control the supply of power to the fixing heaters, and are controlled by temperature signals  408 - 1  and  408 - 2  measured by the temperature sensors  407 - 1  and  407 - 2 . The AC power supplies  406 - 1  and  406 - 2  are power supplies for the fixing heaters. The temperature sensors  407 - 1  and  407 - 2  measure the temperatures of the fixing heaters  403 - 1  and  403 - 2 . The temperature signals  408 - 1  and  408 - 2  are analog data representing the temperatures of the fixing heaters.  
      [Processing to Be Performed by Third Arrangement upon Occurrence of Abnormality in Fixing Device:  FIG. 6 ] 
      A control flow to be executed in the above arrangement comprising the fixing devices  400 - 1  and  400 - 2  and the fixing controller  401  when at least one of the thermal switches  404 - 1  and  404 - 2  of the two fixing devices  400 - 1  and  400 - 2  is turned off upon detection of a temperature equal to or higher than the set upper limit temperature Tmax will be described with reference to  FIG. 6 .  
      If it is determined in step S 201  in  FIG. 6  that the thermal switch  404 - 1  of the first fixing device  400 - 1  is OFF (a temperature equal to or higher than the upper limit temperature Tmax is detected), the flow advances to step S 202 , as is obvious from the arrangement shown in  FIG. 5 . In this step, all the relays  402 - 1  and  402 - 2  are turned off to stop heating the fixing heaters (i.e., the fixing heaters  403 - 1  and  403 - 2 ) of all the fixing devices.  
      If it is determined in step S 201  that the thermal switch  404 - 1  of the fixing device  400 - 1  is ON (any temperature equal to or higher than the upper limit temperature Tmax is not detected), the flow advances to step S 203 . If it is determined in step S 203  that the thermal switch  404 - 2  of the second fixing device  400 - 2  is OFF (a temperature equal to or higher than the upper limit temperature Tmax is detected), the flow advances to step S 202 , as is obvious from the arrangement shown in  FIG. 5 . As a result, all the relays  402 - 1  and  402 - 2  are turned off to stop heating the fixing heaters (i.e., the fixing heaters  403 - 1  and  403 - 2 ) of all the fixing devices.  
      If it is determined in step S 203  that the thermal switch  404 - 2  of the fixing device  400 - 2  is ON (any temperature equal to or higher than the upper limit temperature Tmax is not detected), the flow returns to step S 201 .  
      In this manner, in the third arrangement shown in  FIG. 5 , when the measured temperature of one of the fixing heaters of the fixing devices  400 - 1  and  400 - 2  exceeds the set upper limit temperature Tmax, the thermal switch is turned off, and it is determined that the fixing heater is abnormal, thereby controlling the switches to stop the supply of power to all the fixing devices. Therefore, the image forming apparatus of the present invention can form an image more safely.  
      [Control Arrangement (Fourth Arrangement) for Abnormality Processing for Fixing Devices:  FIG. 7 ] 
      Abnormality processing for each fixing device in the arrangement exemplified in each of  FIGS. 1, 4 , and  5  is performed such that the fixing heaters are turned off when a temperature abnormality is detected. However, this abnormality processing is not limited to that to be performed upon detection of a temperature. For example, as shown in  FIG. 7 , fixing devices and a fixing controller may incorporate serial I/Fs, so that when normal communication cannot be conducted between them, the fixing devices are turned off to ensure the safety of the image forming apparatus. Abnormality processing in this arrangement will be described below.  
       FIG. 7  is a block diagram showing a control system which performs abnormality processing for the fixing devices in the image forming apparatus with the arrangement using serial I/Fs. This apparatus comprises a fixing controller  501  having a control IC  510 - 3  including a serial I/F and two fixing devices  500 - 1  and  500 - 2 . The fixing devices  500 - 1  and  500 - 2  respectively comprise relays  502 - 1  and  502 - 2 , fixing heaters  503 - 1  and  503 - 2 , temperature sensors  504 - 1  and  504 - 2 , comparators  505 - 1  and  505 - 2 , and control ICs  510 - 1  and  510 - 2  including serial I/Fs. The fixing devices  500 - 1  and  500 - 2  are respectively connected to AC power supplies  506 - 1  and  506 - 2  as external power supplies.  
      The fixing controller  5010 N/OFF-controls the two fixing heaters  503 - 1  and  503 - 2  in accordance with information such as the type of printing sheet, the temperatures of the fixing heaters, and fixing error detection information. The fixing controller  501  can control the temperatures of the fixing devices  500 - 1  and  500 - 2  by ON/OFF-controlling the fixing heaters  503 - 1  and  503 - 2 , and turn off the fixing heaters upon occurrence of an abnormality in the fixing devices  500 - 1  and  500 - 2 . The relays  502 - 1  and  502 - 2  are controlled by heater ON/OFF signals  507 - 1  and  507 - 2  to control the supply of power from the AC power supplies  506 - 1  and  506 - 2  to the fixing heaters  503 - 1  and  503 - 2 . The fixing heaters  503 - 1  and  503 - 2  generate heat for fixing toner on a printing sheet. The temperature sensors  504 - 1  and  504 - 2  measure the temperatures of the fixing heaters  503 - 1  and  503 - 2 .  
      The comparators  505 - 1  and  505 - 2  compare measured temperatures from the temperature sensors  504 - 1  and  504 - 2  (the temperatures of the fixing heaters  503 - 1  and  503 - 2 ) with a preset temperature Tmax (predetermined temperature) to determine whether or not the measured temperatures exceed the predetermined temperature. If the measured temperatures exceed the predetermined temperature Tmax, output signals from the comparators  505 - 1  and  505 - 2  are activated to output error signals  1  and  2  indicated by  508 - 1  and  508 - 2  (signals indicating that the fixing heaters  503 - 1  and  503 - 2  are abnormal). If the measured temperatures do not exceed the predetermined temperature Tmax, the comparators  505 - 1  and  505 - 2  do not activate their output signals and hence do not output error signals  1  and  2 .  
      The AC power supplies  506 - 1  and  506 - 2  are power supplies for the fixing heaters. The signals  507 - 1  and  507 - 2  are ON/OFF signals for the relays, and are signals which control the supply of power to the fixing heaters  503 - 1  and  503 - 2 . Reference numerals  509 - 1  and  509 - 2  denote temperature signals, which are analog data representing the temperatures of the fixing heaters; and  510 - 1 ,  510 - 2 , and  510 - 3 , control ICs including serial I/Fs. Data can be exchanged between the control ICs  510 - 1  and  510 - 3  and between the control ICs  510 - 2  and  510 - 3  by serial communication.  
      Note that when the control IC  510 - 1  including the serial I/F receives the error signal  508 - 1  from the comparator  505 - 1 , the control IC  510 - 1  including the serial I/F can stop serial communication with the control IC  510 - 3  including the serial I/F. Likewise, when the control IC  510 - 2  including the serial I/F receives the error signal  508 - 2  from the comparator  505 - 2 , the control IC  510 - 2  including the serial I/F can stop serial communication with the control IC  510 - 3  including the serial I/F. At this time, when the control IC  510 - 3  including the serial I/F detects this abnormal state (e.g., detects that serial communication from the control IC  510 - 1  including the serial I/F to the control IC  510 - 3  including the serial I/F is stopped, or serial communication from the control IC  510 - 2  including the serial I/F to the control IC  510 - 3  including the serial I/F is stopped), all the relays  502 - 1  and  502 - 2  are turned off by using the ON/OFF signals  507 - 1  and  507 - 2  for the relays, thereby stopping the heating of the fixing heaters (i.e., the fixing heaters  503 - 1  and  503 - 2 ) of all the fixing devices.  
      [Processing to be Performed by Fourth Arrangement Upon Occurrence of Abnormality in Fixing Device:  FIG. 8 ] 
       FIG. 8  shows a control flow to be executed in the above arrangement comprising the fixing controller  501  and the two fixing devices  500 - 1  and  500 - 2  when at least one of serial communication between the fixing device  500 - 1  and the fixing controller  501  and serial communication between the fixing device  500 - 2  and the fixing controller  501  cannot be performed. This control is executed while a CPU (not shown) in the image forming apparatus controls the respective components by using RAM (not shown) in accordance with a control program stored in ROM (not shown).  
      First of all, if it is determined in step S 301  in  FIG. 8  that serial communication between the first fixing device  500 - 1  and the fixing controller  501  is abnormal (e.g. when the control IC  510 - 1  including the serial I/F stops sending a response signal to the control IC  510 - 1  including the serial I/F, when a parity check using a parity bit contained in serial transfer data indicates an error, when an error is found by data comparison performed by transmitting the same data a plurality of number of times, or when an error is found by a loopback check (data comparison upon reception of transmitted data without any change)), the flow advances to step S 302 , in which upon detecting the abnormal state (detecting that serial communication from the control IC  510 - 1  including the serial I/F to the control IC  510 - 3  including the serial I/F is stopped), the control IC  510 - 3  turns off all the relays  502 - 1  and  502 - 2  by using the ON/OFF signals  507 - 1  and  507 - 2  for the relays, thereby stopping heating the fixing heaters (i.e., the fixing heaters  503 - 1  and  503 - 2 ) of all the fixing devices.  
      If it is determined in step S 301  that serial communication can be normally done between the first fixing device  500 - 1  and the fixing controller  501 , the flow advances to step S 303 . If it is determined in step S 303  that serial communication between the second fixing device  500 - 2  and the fixing controller  501  is abnormal (when the control IC  510 - 2  including the serial I/F stops sending a response signal to the control IC  510 - 3  including the serial I/F, when a parity check performed by using a parity bit contained in serial transfer data indicates an error, when an error is found by data comparison performed by transmitting the same data a plurality of number of times, or when an error is found by a loopback check (data comparison upon reception of transmitted data without any change)), the flow advances to step S 303 , in which upon detecting the abnormal state (detecting that serial communication from the control IC  510 - 2  including the serial I/F to the control IC  510 - 3  including the serial I/F is stopped), the control IC  510 - 3  including the serial I/F turns off all the relays  502 - 1  and  502 - 2  by using the ON/OFF signals  507 - 1  and  507 - 2  for the relays, thereby stopping heating the fixing heaters (i.e., the fixing heaters  503 - 1  and  503 - 2 ) of all the fixing devices.  
      If it is determined in step S 303  that the control IC  510 - 3  including the serial I/F does not detect the abnormal state, the flow returns to step S 301 .  
      In this manner, in the fourth arrangement shown in  FIG. 7 , when at least one of serial communication between the fixing device  500 - 1  and the fixing controller  501  and serial communication between the fixing device  500 - 2  and the fixing controller  501  cannot be performed because of a failure in one of the fixing devices  500 - 1  and  500 - 2 , it is discriminated that an abnormality has occurred in one of the fixing devices, and the switches are controlled to stop the supply of power to all the fixing devices. This allows the image forming apparatus of the present invention to form an image more safely.  
     Other Embodiment  
      The present invention can take embodiments as, for example, an apparatus, method, program, and storage medium. More specifically, the present invention may be applied to a system constituted by a plurality of devices (e.g., a host computer, an interface device, a reader, a printer, and the like) or an apparatus comprising a single device (e.g., a copying machine, a facsimile apparatus, or the like).  
      The object of the present invention is realized even by supplying a storage medium (or a recording medium) storing software program codes for realizing the functions of the above-described embodiment to a system or apparatus, and causing the computer (or a CPU or MPU) of the system or apparatus to read out and execute the program codes stored in the storage medium. In this case, the program codes read out from the storage medium realize the functions of the above-described embodiment by themselves, and the storage medium storing the program codes constitutes the present invention.  
      The functions of the above-described embodiment are realized not only when the readout program codes are executed by the computer but also when the OS (Operating System) running on the computer performs part or all of actual processing on the basis of the instructions of the program codes.  
      The functions of the above-described embodiment are also realized when the program codes read out from the storage medium are written in the memory of a function expansion board inserted into the computer or a function expansion unit connected to the computer, and the CPU of the function expansion board or function expansion unit performs part or all of actual processing on the basis of the instructions of the program codes. When the present invention is to be applied to the above storage medium, program codes corresponding to the above-described flowcharts (shown in  FIGS. 2, 6 , and  8 ) are stored in the storage medium.  
      As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the claims.  
     CLAIM OF PRIORITY  
      This application claims priority from Japanese Patent Application No. 2004-217771 filed on Jul. 26, 2004, which is hereby incorporated by reference herein.