Abstract:
A fixing unit fixes a developing material deposited on a recording medium by heating and pressing the recording medium. The fixing unit includes two heating members that apply heat to the recording medium and two temperature-sensitive shutoff devices. The two temperature-sensitive shutoff devices are connected in series with a corresponding one of the heating members. Each of the temperature-sensitive shut-off devices includes a first heat-sensitive shut-off element and a second heat-sensitive shut-off element. The first heat-sensitive shut-off element shuts off electric power to the corresponding one of the two heating members when a temperature of a surface of the corresponding one of the two heating members exceeds a certain value. The second heat-sensitive shut-off element shuts off electric power to the corresponding one of the two heating members when a temperature of a surface of the second one of the two heating members exceeds a certain value.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of copending U.S. application Ser. No. 10/153,534, filed May 21, 2002, now U.S. Pat. 6,597,879 entitled “FIXING UNIT,” the entire disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fixing unit incorporated in an electrophotographic recording apparatus, and more particularly to a fixing unit where the developer material deposited on a print medium is pressed and heated to fuse. 
     2. Description of the Related Art 
     A conventional fixing unit for use in an electrophotographic printer includes a rotating heat roller and a rotating backup roller. The surfaces of the heat roller and backup roller have a rubber material or a resin material wrapping around them. The heat roller is cylindrical and has a built-in heater in the form of, for example, a halogen lamp. Electric power is supplied to the heater, which in turn generates heat to heat the heat roller to a desired temperature. 
     The heat roller has a temperature sensor in the form of a thermistor. The temperature sensor detects the temperature of the surface of the heat roller. The detection signal causes a control circuit to turn on and off the electric power supplied to the halogen lamp, thereby maintaining the surface temperature of the heat roller to a substantially constant value. For safety of the system, there is provided a thermostat that shuts off electric power when the feedback control operates abnormally to overheat the heat roller. The thermostat shuts off the electric power before the temperature of the heat roller exceeds a maximum allowable value, thereby preventing an abnormal increase in temperature. 
     FIG. 6 illustrates another conventional fixing unit. Referring to FIG. 6, instead of a heat roller and a backup roller, a fixing unit  1  uses two heat rollers  2  and  3  that heat a print medium both from the front side and from the back side simultaneously. This type of fixing unit is advantageous when the printing speed of the electrophotographic printer is to be increased. The recording medium passes through the fixing unit at a high speed and therefore heat rollers must apply a sufficient amount of heat to the printing medium in a short time during which the printing medium passes through the fixing unit. 
     There are provided thermistor sensors  4  and  5  on the heat rollers  2  and  3 , respectively. The thermistor sensors  4  and  5  are connected to control circuits  8  and  9  through connectors  6  and  7 , respectively. The control circuits  8  and  9  are connected to an a-c main line  12  through thyristors  10  and  11 . 
     Thermostats  13  and  14  are disposed on the surfaces of the heat rollers  2  and  3 , respectively. The thermostat  13  has one cord connected to a halogen lamp  15  in the heat roller  2  and the other cord connected to the a-c main line  12  through a fuse  16  and the connector  6 . Likewise, the thermostat  14  has one cord connected to a halogen lamp  17  in the heat rollers  3  and the other cord connected to the AC main line  12  through the fuse  16 . 
     FIG. 7 is a graph that illustrates changes in the surface temperature of the heat rollers in the conventional art when temperature control fails. Curve A indicates the surface temperature of the heat roller  22  when temperature control fails and Curve B shows the surface temperature of the heat roller  23  when temperature control operates normally. 
     The operation of the conventional fixing unit of the aforementioned construction will be described. If the temperature control involving one of the thermistor sensors  4  and  5  should fail and a heat roller in a failed control system is overheated, a corresponding thermostat operates to shut off the circuit before the temperature reaches a tolerable value. The temperature of the heat roller  23  under abnormal temperature control will start to decrease. The heat roller  22  under normal temperature control maintains its surface temperature substantially at the target value. Therefore, as shown by Curve A, the temperature of the heat roller  23  will not decrease rapidly but slowly reach the target value at time t 3 . 
     With the aforementioned conventional fixing unit that employs two heat rollers, the thermostats are connected to separate circuits. If one of the feedback control systems fails, a corresponding thermostat in the failed system is shut off. A thermostat in the normally operating system is not shut off but performs its on and off operation under the control of the output of a corresponding thermistor. In other words, the surface of a normally operating heat roller is maintained at a desired temperature. The temperature of an abnormally operating heat roller will not decrease and the abnormal condition will remain for a long time. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a fixing unit in which when a heat roller is overheated due to an abnormal condition, the abnormal condition is prevented from lasting for a long time. 
     A fixing unit fixes a developing material deposited on a recording medium by heating and pressing the recording medium. The fixing unit includes two heating members that apply heat to the recording medium and two temperature-sensitive shutoff devices. The two temperature-sensitive shutoff devices are connected in series with a corresponding one of the heating members. Each of the temperature-sensitive shut-off devices includes a first heat-sensitive shut-off element and a second heat-sensitive shut-off element. The first heat-sensitive shut-off element shuts off electric power to the corresponding one of the two heating members when a temperature of a surface of the corresponding one of the two heating members exceeds a certain value. The second heat-sensitive shut-off element shuts off electric power to the corresponding one of the two heating members when a temperature of a surface of the second one of the two heating members exceeds a certain value. 
     A fixing unit fixes a developing material deposited on a recording medium by heating and pressing the recording medium. The fixing unit includes a first number of heating members that apply heat to the recording medium and a second number of temperature-sensitive shut-off devices in series with each one of the first number of heating members. Each of the first number of heating elements receives electric power through a series connection of the second number of switches. The second number of temperature-sensitive shut-off devices operates in such a way that each of the first number of heating elements receives electric power through the series connection of the second number of temperature-sensitive shut-off devices. Each of the second number of temperature-sensitive shut-off devices receives heat from a surface of a corresponding one of the first number of heating members to turn off the electric power in response to a temperature of the surface. 
     A fixing unit fixes a developing material deposited on a recording medium by heating and pressing the recording medium. The fixing unit includes a first number of heating members that apply heat to the recording medium and a second number of switches. Each of the first number of heating members receives electric power through a series circuit of the second number of switches. Each of the second number of switches responds to a surface temperature of a corresponding one of the first number of heating members so that when the surface temperature exceeds a predetermined value, the electric power is shut off. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a schematic diagram, illustrating a fixing unit according to a first embodiment of the invention; 
     FIG. 2 is a graph that illustrates changes in the surface temperature of the heat roller in the first embodiment; 
     FIG. 3 is a schematic diagram, illustrating a fixing unit according to a second embodiment; 
     FIG. 4 is a cross-sectional view of the fixing unit of FIG. 3; 
     FIG. 5 illustrates a fixing unit according to a third embodiment; 
     FIG. 6 illustrates a conventional fixing unit; and 
     FIG. 7 is a graph that illustrates changes in the surface temperature of the heat roller in the conventional fixing unit when temperature control fails. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment 
     FIG. 1 is a schematic diagram, illustrating a fixing unit according to a first embodiment of the invention. 
     Referring to FIG. 1, a fixing unit  21  includes two heat rollers  22  and  23 . The surfaces  24  and  25  of the heat rollers  22  and  23  are covered with a layer of a rubber material or a resin material. Halogen lamps  26  and  27  are disposed in the heat rollers  22  and  23 , respectively, and serve as a heater. The halogen lamp  26  has one end  26   a  connected to a cord  28  and the other end  26   b  connected to a cord  29 . The halogen lamp  27  has one end  27   a  connected to a cord  30  and the other end  27   b  connected to the cord  29 . 
     Thermistor sensors  4  and  5  are disposed on the heat rollers  22  and  23 , respectively, and connected to control circuits  8  and  9  through connectors  6  and  7 , respectively. 
     Thermostats  13  and  14  are disposed on the surfaces of the heat rollers  22  and  23 . The thermostats  13  and  14  take the form of an overtemperature thermostat. 
     The cord  28  connects the halogen lamp  26  and a cathode of a thyristor  10  through a connector  6 . An anode of the thyristor  10  is connected to one of the terminals of an a-c main line  12 . The cord  30  connects the halogen lamp  27  and the cathode of a thyristor  11  through a connector  7 . The anode of the thyristor  11  is connected to one of the terminals of the a-c main line  12 . The cord  32  connects the thermostat  13  to the main line  12  through the connector  6  and fuse  16 . The thermostat  14  is connected to the halogen lamps  26  and  27  through the cord  29 . The thermostat  14  is also connected to the thermostat  13  through the cord  31 . The aforementioned circuit connection completes a series connection between the thermostat  13  and thermostat  14 . 
     Temperature-controlling circuits  8  and  9  are connected to a temperature-setting circuit  34  and to gates of the thyristors  10  and  11 . The temperature-setting circuit  34  sends a command to the temperature-controlling circuits  8  and  9  to control the temperatures of the heat rollers  22  and  23 , respectively. 
     The operation of the first embodiment will be described. When the printer is powered on and a printing operation is initiated, the temperature-setting circuit  34  sends a temperature-setting command to the temperature-controlling circuits  8  and  9  to set the surfaces of the heat rollers  22  and  23  to a target temperature. The temperature-controlling circuit  8  compares a detection signal from the thermistor sensor  4  with a target temperature. If the detection signal is lower than the target temperature, then the temperature-controlling circuit  8  provides a signal to the gate of the thyristor  10  to turn on the thyristor  10 . Then, the thyristor  10  allows a-c current to flow therethrough, the a-c current flowing through the cord  28  into the halogen lamp  26  to heat the heat roller  22 . 
     The thermistor  4  monitors the surface temperature of the heat roller  22 . If the temperature monitored by the thermistor  4  exceeds a threshold value, then the temperature-controlling circuit  8  provides a signal to the gate of the thyristor  10 , thereby turning off the thyristor  10 . In response to the signal, the thyristor  10  shuts off the a-c current flowing through it, so that no current flows through the halogen lamp  26  and therefore the heat roller  22  begins to cool down. If the surface temperature of the heat roller  22  decreases below the threshold value, the aforementioned operation is performed so that current flows through the halogen lamp  26  again. By repeating the aforementioned operation, the surface temperature of the heat roller  22  is maintained substantially to a target temperature. 
     A similar temperature control is performed for the heat roller  23 . That is, the temperature-controlling circuit  9  compares a detection signal from the thermistor sensor  5  with a target temperature received from the temperature-setting circuit  34 . The comparison result is used to drive the thyristor  11  to control the current flowing through the halogen lamp  27 , so that the surface temperature of the heat roller  23  is maintained substantially to the target temperature. 
     During normal operation, the a-c currents flowing out of the halogen lamps  26  and  27  are added together at the terminal of the thermostat  14  and then further flows through the thermostat  13 , connected in series with the thermostat  14 , to the fuse  16 . 
     If the feedback control through the thermistor sensors  4  and  5  should fail so that the halogen lamps  26  and  27  are overheated, the thermostat in the circuit having an overheated halogen lamp operates to shut off the electric power supplied thereto. For example, if a foreign matter is caught between the thermistor sensor  4  and the heat roller  22 , the foreign matter prevents the thermistor sensor  4  from detecting the surface temperature of the heat roller  22  properly. As a result, a large current flows through the halogen lamp  26  and may cause the surface temperature of the heat roller  22  to exceed a target value. 
     When the surface of the heat roller  22  exceeds the upper limit temperature, the thermostat  13  operates to shut off the current flowing through the halogen lamp  26 . Therefore, the current flowing through the halogen lamp  27  is also shut off. Shutting off the current that flows through the halogen lamps  26  and  27  causes the surface temperature of the heat rollers  22  and  23  to rapidly decrease. 
     FIG. 2 is a graph that illustrates changes in the surface temperature of the heat roller in the first embodiment. 
     Referring to FIG. 2, Curve A indicates the surface temperature of the heat roller  22  and Curve B shows the surface temperature of the heat roller  23 . Upper limit temperature is a temperature beyond which the thermostat  13  operates to shut off the current through the circuit and target temperature is a temperature value toward which the surface temperatures of the heat rollers  22  and  23  are controlled. As shown in FIG. 2, when the surface temperature of the heat roller  22  increases to the upper limit temperature at time t 1 , the thermostat  13  operates to shut off the current through it, as well as the current flowing through the halogen lamp  27  in the heat roller  23 . Thus, the surface temperature of the heat roller  23  also decreases. The decrease in the surface temperature of the heat roller  23  allows the surface temperature of the heat roller  22  to decrease promptly. In fact, the time required for the heat roller  22  to cool down to the target temperature is t 2  in FIG. 2, shorter than t 3  in FIG.  7 . As described previously, the thermostats  13  and  14  take the form of an overtemperature thermostat. That is, the thermostat opens at, for example, 150° C. and closes at, for example, below 0° C. Thus, once the thermostat opens at a high abnormal temperature, the circuit will remain open after the heat rollers cool down to room temperature. The use of an overtemperature thermostat enhances safety of the apparatus. A thermal fuse may be used in place of the overtemperature thermostat. 
     As described above, the thermostats  13  and  14  are connected in series with a parallel circuit of the halogen lamps  26  and  27 . Therefore, when a failure of the temperature control for one of the heat rollers  22  and  23  causes a corresponding heat roller to be overheated, a corresponding thermostat operates to shut off the current flowing through the halogen lamps  26  and  27 . This makes an abnormal condition to quickly terminate, thereby improving safety of the fixing unit  21 . 
     Second Embodiment 
     While the first embodiment has been described with respect to a fixing unit having two heat rollers, more heat rollers may be employed. A second embodiment differs from the first embodiment in that the fixing unit uses three heat rollers. 
     FIG. 3 is a schematic diagram, illustrating a fixing unit according to the second embodiment. 
     FIG. 4 is a cross-sectional view of the fixing unit of FIG.  3 . 
     Referring to FIG. 3, a heat roller  41  is in contact with the heat roller  22 . A thermistor sensor  42  is disposed on a surface  82  of the heat roller  41 . The thermistor sensor  42  is electrically connected through a connector  43  to a temperature-controlling circuit  44 . A thermistor sensor  116  is disposed on the surface of the heat roller  22 . The thermistor  116  is electrically connected through a connector  6  to a temperature-controlling circuit  8 . The temperature-controlling circuit  44  is connected to the gate of a thyristor  45  and the temperature-setting circuit  34 . 
     A thermostat  46  is disposed on the surface  82  of the heat roller  41 . A cord  47  connects the thermostat  46  to the thermostat  13 , and a cord  48  connects the thermostat  46  to the a-c main line  12  through the connector  43  and the fuse  16 . 
     A halogen lamp  49  has one end  49   b  thereof connected to one terminal of the thermostat  14  through a cord  29 , and the other end  49   a  thereof connected to the cathode of the thyristor  45  through a cord  50  and the connector  43 . The anode of the thyristor  45  is connected to the another terminal of the a-c main line  12 . The rest of the construction is the same as the first embodiment. 
     Referring to FIG. 4, the heat rollers  22 ,  23 , and  41  rotate in directions shown by arrows A, B, and C and the recording medium travels in a direction shown by arrow D. The positional relation between rollers  23  and  41  and the roller  22  may be interchanged so that the recording medium travels upward after it passes through between the roller  23  and the roller  22 . An angle θ and the diameters of the rollers can be selected by considering the flexibility of the recording medium. This type of fixing unit is advantageous when high-speed printing is performed. When the recording medium passes through the fixing unit at a high speed, the recording medium cannot receive a sufficient amount of heat. The configuration of FIG. 4 effectively increases the time during which the recording medium is subjected to heating. The recording medium is guided by a guide member, not shown, that extends substantially through the angle θ in such a way that the recording medium passes between the heat roller  22  and heat roller  41  and then between the heat roller  22  and heat roller  23 . 
     When an abnormal condition occurs in any one of the three heat rollers  41 ,  22 , and  23 , the current flowing through the halogen lamps  49 ,  113 , and  27  in all other heat rollers is shut off, thereby preventing the abnormal condition from lasting for a long time. The second embodiment has been described with respect to three rollers  22 ,  23 , and  41  rotate in contact with the recording medium. An additional heat roller may be combined to these heat rollers  22 ,  23 , and  41  so that the additional heat roller rotates in contact with one of the three rollers  22 ,  23 , and  41 , and heats the recording medium indirectly. 
     Third Embodiment 
     FIG. 5 illustrates a fixing unit according to a third embodiment. 
     With the first and second embodiments, the thermostats are simply connected in series regardless of the number of heat rollers. The third embodiment differs from the first and second embodiments in that there are as many series-connections of thermostats as there are heat rollers. 
     Referring to FIG. 5, a fixing unit  51  according to the third embodiment includes two heat rollers  52  and  53 . The heat rollers have surfaces  54  and  55 , respectively, which are covered with a rubber material or a resin material. The heat rollers  52  and  53  have halogen lamps  56  and  57  built therein, respectively. The halogen lamps  56  and  57  have one ends thereof connected to cords  58  and  59  and the other ends thereof connected to cords  60  and  61 . 
     There are provided the thermistor sensors  4  and  5  on the surface of the heat rollers  52  and  53 , respectively. The thermistor sensors  4  and  5  are connected to temperature-controlling circuits  64  and  65  through connectors  62  and  63 , respectively. Thermostats  66  and  67  are disposed on the surface of the heat roller  52 , and thermostats  68  and  69  are disposed on the heat roller  53 . 
     A first series circuit is formed as follows: A cord  58  connects the halogen lamp  56  in the heat roller  52  to the thermostat  69  on the heat roller  53 . A cord  70  connects the thermostat  69  to the thermostat  66  on the heat roller  52 . A cord  71  connects the thermostat  66  to an a-c main line  73  through the connector  62  and a fuse  72 . The cord  59  connects a cathode of a thyristor  74  through the connector  62  to the halogen lamp  56 . The thyristor  74  has an anode connected to the a-c main line  73 . 
     Another series circuit is formed as follows: A cord  60  connects the halogen lamp  57  in the heat roller  53  to the thermostat  67  on the heat roller  52 . A cord  75  connects the thermostat  67  to the thermostat  68  on the heat roller  53 . A cord  76  connects the thermostat  68  to the a-c main line  73  through the connector  63  and the fuse  72 . A thyristor  77  has a cathode connected through the cord  61  and the connector  63  to the halogen lamp  57 , and an anode connected to the a-c supply  73 . 
     The current that flows through the thermostats  66  and  69  is equal to the current that flows through the halogen lamp  56 . The current that flows through the thermostats  67  and  68  is equal to the current that flows through the halogen lamp  57 . 
     The thermostats  66  and  67  disposed on the heat roller  52  are designed to operate at substantially the same temperature. Likewise, the thermostats  68  and  69  disposed on the heat roller  53  are designed to operate at substantially the same temperature. 
     The temperature controlling circuits  64  and  65  are connected to a temperature-setting circuit  78  and gates of the thyristors  74  an  77 . The temperature-setting circuit  78  provides a command signal to the temperature-controlling circuits  64  and  65  to perform the temperature control for the heat rollers  52  and  53 . 
     The operation of the third embodiment will now be described. In response to the command signal from the temperature-setting circuit  78 , the temperature-controlling circuit  64  sends an ON signal to the thyristor  74 . The thyristor  74  then operates to allow an a-c current to flow through the halogen lamp  56  in the heat roller  52 . The current that flows through the halogen lamp  56  also flows through the thermostat  69  on the heat roller  53 , then through the thermostat  66  on the heat roller  52 , and finally returns to the a-c main line  73  through the connector  62  and fuse  72 . 
     In response to the command signal from the temperature-setting circuit  78 , the temperature-controlling circuit  65  sends an ON signal to the thyristor  77 . The thyristor  77  then operates to allow an a-c current to flow through the halogen lamp  57  in the heat roller  53 . The current that flows through the halogen lamp  57  also flows through the thermostat  67  on the heat roller  52 , then the thermostat  68  on the heat roller  52 , and finally returns to the a-c main line  73  through the connector  63  and fuse  72 . 
     If the feedback control through one of the thermistor sensors should fail to properly operate and a corresponding heat roller is overheated, the thermostat on the overheated heat roller operates to shut off electric power through it. For example, when the heat roller  52  is overheated, the thermostats  66  and  67  operate simultaneously or substantially simultaneously to shut off the current flowing through the halogen lamps  56  and  57 . Likewise, when the heat roller  53  is overheated, the thermostats  68  and  69  operate simultaneously or substantially simultaneously to shut off the current flowing through the halogen lamps  56  and  57 . 
     For example, if a foreign material is trapped between the thermistor  4  and the heat roller  52 , the foreign matter prevents the thermistor sensor  4  from detecting the surface temperature of the heat roller  52  properly. As a result, a large current flows through the halogen lamp  56  and may cause the surface temperature of the heat roller  52  to exceed the upper limit temperature. 
     If the surface temperature of the heat roller  52  exceeds the upper limit temperature, the thermostats  66  and  67  operate. In other words, the thermostat  66  shuts off the current flowing through the halogen lamp  56  while the thermostat  67  shuts off the current flowing through the halogen lamp  57 . Shutting off the currents that flow through the halogen lamps  56  and  57  allows the heat rollers  52  and  53  to cool down rapidly. 
     According to the third embodiment, when the temperature control for one of a plurality of heat rollers fails and causes the associated heat roller to be overheated, the system operates to shut off not only the current flowing through the halogen lamp for the heat roller under failed temperature control but also the current flowing through the other halogen lamps for the heat rollers under normal temperature control. This way of operation prevents the abnormal condition from lasting a long time and improves safety of the system. 
     The circuit is configured in such a way that the current that flows through the respective thermostats is equal to the current for one halogen lamp. This allows employing inexpensive thermostats having a low current rating. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.