Patent Publication Number: US-6993262-B2

Title: Fixing apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-215776, filed Jul. 24, 2002, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a fixing apparatus designed for use in an image forming apparatuses such as copiers or printers and configured to fix developer images on paper sheets. 
   2. Description of the Related Art 
   Any image forming apparatus utilizing digital technology, such as an electronic copier, comprises a fixing apparatus. The fixing apparatus has a heating roller and a pressing roller. The pressing roller contacts the heating roller. A paper sheet with a toner image on it can be fed forward through the nip between the heating roller and the pressing roller. As the sheet is so fed, the toner image is fixed on the paper sheet by virtue of the heat generated by the heating roller. 
   The heating roller generates heat by means of induction heating achieved by a high-frequency magnetic field. The heating roller incorporates a coil and a capacitor connected to the coil. The coil and capacitor constitute a resonance circuit. When the resonance circuit is excited, a high-frequency current flows in the coil. As the current flows in the coil, the coil generates a high-frequency magnetic field. The magnetic field induces an eddy current in the heating roller. The heating roller generates Joule heat from the eddy current. 
   In any fixing apparatus that performs induction heating is performed, a thermostat is attached to the heating roller and is provided on the power-supply line that connects the fixing apparatus to the commercially available power supply. When the thermostat operates, the power-supply line is electrically cut and the induction heating stops. This prevents the heating roller from being heated to an excessive temperature. 
   The thermostat cannot operate the moment an excessive heating of the heating roller is detected. Rather, it starts with some time delay. Consequently, the heating roller and its neighboring components may be thermally influenced. 
   A part of the power line goes around the thermostat provided on the heating roller. This arrangement of the power line is undesirable in view of noise reduction and operation safety. 
   BRIEF SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a fixing apparatus that excels in safety and reliability, in which an excessive temperature rise of the heating roller can be prevented, without any delay, without using a thermostat and without arranging the power line around any component. 
   A fixing apparatus according to this invention comprises: a heating member to be rotated to fix toner images; a coil which generates a high-frequency magnetic field to perform induction heating in the heating member; a high-frequency wave generating circuit which operates with power supplied from a power supply and which outputs a high-frequency current to the coil for generating the high-frequency magnetic field; a current-detecting unit which detects a current supplied from the power supply to the high-frequency wave generating circuit; a control unit which drives the high-frequency wave generating circuit when an operation-on signal for initiating the induction heating acquires a first logic high level and which stops the high-frequency wave generating circuit when the operation-on signal acquires a second logic level; and a protection circuit which controls supply of the current to the high-frequency wave generating circuit in accordance with the logic level of the operation-on signal and the magnitude of the current detected by the current-detecting unit. 
   Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
       FIG. 1  is a diagram showing the major components of a fixing apparatus that is one embodiment of the invention; 
       FIG. 2  is a block diagram of the control circuit incorporated in an electronic copier relating to the embodiment; 
       FIG. 3  is a block diagram of the electric circuit provided in the embodiment; and 
       FIG. 4  is a flowchart explaining the operation of the print CPU provided in the embodiment. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An embodiment of the present invention will be described with reference to the accompanying drawings. 
   An image forming apparatus according to this invention is, for example, an electronic copier. The copier comprises a scanning unit, a process unit, and a fixing apparatus. The scanning unit optically reads the image printed on an original. The processing unit (unit  95 , described later) forms, on a paper sheet, a toner image corresponding to the image read by the scanning unit. The fixing apparatus (apparatus  100 , described later) heats the paper sheet, thereby fixing the toner image on the paper sheet. The structure of this image forming apparatus is disclosed in U.S. patent application Ser. No. 09/955,089 and will not be described in detail. 
     FIG. 1  depicts the fixing apparatus  100 . As shown in  FIG. 1 , the fixing apparatus  100  comprises a heating roller  101  and a pressing roller  102 . The heating roller  101  is located above the copy-sheet path. The pressing roller  102  lies below the copy-sheet path and contacts the heating roller  101 , pressed onto the roller  101  by means of a pressing mechanism (not shown). The contacting parts of the rollers  101  and  102  form a nip. The nip has a prescribed length. 
   The heating roller  101  comprises a hollow cylinder and a layer. The cylinder is made of electrically conductive material, for example iron. The layer is made of, for example, Teflon, and covers the outer circumferential surface of the hollow cylinder. The heating roller  101  can be rotated clockwise in FIG.  1 . The pressing roller  102  can be rotated counter-clockwise. A copy sheet S may pass through the nip between the heating roller  101  and the pressing roller  102 . While passing through the nip, the sheet S receives heat from the heating roller  101 . The toner image T on the sheet S is thereby fixed. 
   A sheet-peeling claw  103 , a cleaning member  104 , and a release-agent applying roller  105  are arranged around the heating roller  101 . The sheet-peeling claw  103  is designed to peel a copy sheet S from the heating roller  101 . The cleaning member  104  is configured to remove residual toner, paper dust and the like from the heating roller  101 . The release-agent applying roller  105  is provided to apply a release agent to the outer circumferential surface of the heating roller  101 . 
   The heating roller  101  incorporates a coil  111  that performs induction heating. The coil  111  is wound and held around a core  112 . It is designed to generate a high-frequency magnetic field to achieve induction heating. When the coil  111  generates a high-frequency magnetic field, an eddy current is induced in the heating roller  101 . The roller  101  generates Joule heat from the eddy current. 
   The heating roller  101  that is used as a heating member can be replaced by a belt made of electrically conductive material. The coil  111  may be arranged outside the heating roller  101 , not in the roller  101  as shown in FIG.  1 . 
     FIG. 2  shows the control circuit incorporated in the electronic copier according to the invention. As  FIG. 2  shows, the control circuit comprises a main CPU  50 , a scan CPU  70 , a control-panel CPU  80 , and a print CPU  90 . The CPUs  70 ,  80  and  90  are connected to the main CPU  50 . 
   A ROM  91 , a RAM  92 , a print engine  93 , a sheet-feeding unit  94 , a processing unit  95 , and the fixing apparatus  100  are connected to the print CPU  90 . The ROM  91  stores control programs. The RAM  92  is provided to store data. 
     FIG. 3  depicts the electric circuit of the fixing apparatus  100 . As  FIG. 3  shows, a switch  201  having normally closed contacts  201   a  and  201   b  connects a variable resistor  202  and rectifying circuit  203  to the commercially available power supply  200 . The output of the rectifying circuit  203  is connected to a smoothing capacitor  203 . The smoothing capacitor  204  is connected to a resonant circuit that comprises the above-mentioned coil  111  and a capacitor  205 . A switching element (transistor)  206  is provided on the current path to the resonant circuit to excite the resonant circuit. The switching element  206  is turned on or off by a drive signal supplied from a drive circuit  207 . When the element  206  is turned on, the resonant circuit is excited, inducing a high-frequency current in the coil  111 . As a result, the coil  111  generates a high-frequency magnetic field. 
   The variable resistor  202 , rectifying circuit  203 , smoothing capacitor  204 , capacitor  205  and switching element  206  constitute a high-frequency wave generating circuit that supplies a high-frequency current to the coil  111 . A current-detecting circuit  210  is provided on the current path (power-supply line) extending between the commercially available power supply  200  and the high-frequency wave generating circuit. 
   The current-detecting circuit  210  comprises a voltage-lowering transformer  211 , rectifying circuit  212  and a parallel circuit. The transformer  211  has its primary winding connected to the current path to the high-frequency wave generating circuit. The rectifying circuit  212  is connected to the secondary winding of the transformer  211 . The parallel circuit comprises a resistor  213  and a smoothing capacitor  214 , both connected to the output of the rectifying circuit  212 . The current-detecting circuit  210  outputs a DC voltage that corresponds to the current input to the high-frequency wave generating circuit. The DC voltage is applied to the IH CPU  215 , which is a control unit. 
   The IH CPU  215  receives an output of the coil  111  from the current detected by the current-detecting circuit  210  while the operation-on signal supplied from the print CPU  90  remains at logic “1” level. The IH CPU  215  controls the drive circuit  207 , which drives the high-frequency wave generating circuit so that the output of the coil  111  may remain at a predetermined value. 
   A temperature sensor  301  is mounted on the outer circumferential surface of the heating roller  101 . The temperature sensor  301  receives a DC voltage Vd via resistors  302  and  303 . Hence, the voltage generated across the resistor  303  rises as the temperature of the heating roller  101  rises, decreasing the resistance of the temperature sensor  301 . 
   The voltage generated across the resistor  303  is applied to the print CPU  90  as a signal that represents the temperature the sensor  301  has detected. 
   The voltage generated across the resistor  302  is applied to the negative (−) input terminal of a comparator  304 , too. The DC voltage Vd is applied to a series circuit comprising resistors  305  and  306 . The voltage generated across the resistor  306  is applied as reference voltage to the positive (+) input terminal of the comparator  304 . The output of the comparator  304  is at logic “1” level as long as the voltage generated across the resistor  303  remains lower than the reference voltage. It falls to logic “0” level when the voltage generated across the resistor  303  becomes equal to or higher than the reference voltage (or when the temperature of the heating roller  101  rises to excess). The output of the comparator  304  is input to one input terminal of an AND circuit  307 . 
   The other input terminal of the AND circuit  307  receives the operation-on signal (at logic “1” level) from the print CPU  90 . The operation-on signal supplied from the CPU  90  to the AND circuit  307  is at logic “1” level or logic “0” level in order to maintain the temperature of the heating roller  101  (i.e., the temperature detected by the sensor  301 ) at a preset temperature. As shown in the flowchart of  FIG. 4 , it is determined in Step S 101  whether the temperature detected by the sensor  301  is lower than the preset temperature. If YES, the operation-on signal is set at logic “1” level in Step S 102 . If NO in Step S 101 , the operation-on signal is set at logic “0” level in Step S 103 . 
   The AND circuit  307  outputs the operation-on signal at logic “1” level when the output of the comparator  304  is at logic “1” level. The operation-on signal at logic “1” level is supplied to the IH CPU  215 . 
   In the meantime, the output voltage of the current-detecting circuit  210  is input to the positive (+) input terminal of a comparator  401 . The DC voltage Vd is applied to a series circuit that comprises resistors  402  and  403 . A voltage generated across the resistor  403  is applied as reference voltage to the negative (−) input terminal of the comparator  401 . The output of the comparator  401  is at logic “0” level as long as the output voltage of the current-detecting circuit  210  remains lower than the reference voltage. When the output voltage of the circuit  210  becomes equal to or higher than the reference voltage, the output of the comparator  401  rises to logic “1” level. The output of the comparator  401  is supplied to one input terminal of an AND circuit  404 . The other input terminal of the AND circuit  404  receives an operation-on signal (at logic “1” level) supplied from the AND circuit  307  through an inverter  405 . 
   The comparator  401 , resistors  402  and  403 , AND circuit  404 , inverter  405  and switch  201  constitute a protection circuit  400 . If the AND circuit  307  outputs no operation-on signal and the output of the inverter  405  therefore remains at logic “1” level, the output of the AND circuit  404  rises to logic “1” level when the current-detecting circuit  210  detects the input current and its output voltage rises to the reference voltage or any voltage higher than the reference voltage. The output of the AND circuit  404 , which is at logic “1” level, opens the normally closed contacts  201   a  and  201   b  of the switch  201 . As a result, the current path (power-supply line) to the commercially available power supply is electrically disconnected from to the high-frequency wave generating circuit. 
   How the fixing apparatus described above operates will be explained below. 
   The print CPU  90  outputs an operation-on signal at logic “1” level, which is supplied to the IH CPU  215  via the AND circuit  307 . As long as the operation-on signal remains at logic “1” level, the IH CPU  215  keeps driving the drive circuit  207 . Thus, the drive circuit  207  drives the high-frequency wave generating circuit, which generates power. The power excites the resonant circuit that comprises the coil  111  and capacitor  205 . That is, a high-frequency current flows in the coil  111 , which generates a high-frequency magnetic field. The high-frequency magnetic field induces an eddy current in the heating roller  101 . The heating roller  101  generates Joule heat from the eddy current. 
   When the temperature sensor  301  detects that the temperature of the heating roller  101  becomes too high, the output of the comparator  304  acquires logic “1” level. At the same time the AND circuit  307  blocks the operation-on signal input from the print CPU  90 . Therefore, the operation-on signal is not supplied to the IH CPU  215 . The IH CPU  215  stops controlling the high-frequency wave generating circuit. The heating roller  101  becomes no longer heated too much. 
   The current-detecting circuit  210  detects the input current to the high-frequency wave generating circuit while the high-frequency wave generating circuit is operating. The circuit  210  generates a signal that represents the input current detected. The signal is supplied to the IH CPU  215 . The IH CPU  215  determines the output of the coil  111  from the current that the circuit  210  has detected. The IH CPU  215  controls the high-frequency wave generating circuit to cause the output of the coil  111  to have a prescribed value. 
   The print CPU  90  monitors the output signal of the detector  301 , which represents the temperature of the heating roller  102 . Based on the magnitude of the output signal of the detector  301 , the print CPU  90  keeps or stops outputting the operation-on signal to maintain the temperature of the heating roller  101  at a preset value. When the CPU  90  stops outputting the operation-on signal, the high-frequency wave generating circuit stops operating. 
   Nonetheless, the IH CPU  215  may malfunction to keep driving the high-frequency wave generating circuit, even if the print circuit  90  has stopped generating the operation-on signal. In this case, a current continues to flow from the commercially-available power supply to the high-frequency wave generating circuit. The current-detecting circuit  210  detects this current. The protection circuit  400  processes the output of the circuit  210  and the operation-on at logic “1” level, generating an output at logic “1” level. The output of the protection circuit  400 , which is at logic “1” level, opens the normally-closed contacts  201   a  and  201   b . As a result, the supply of a current to the high-frequency wave generating circuit is stopped. The high-frequency wave generating circuit is therefore immediately stopped. 
   As described above, the current-detecting circuit  210  detects whether the high-frequency wave generating circuit is unnecessarily operating while the operation-on signal remains at logic “0” level. The high-frequency wave generating circuit is immediately stopped before the heating roller  101  is heated to excess, when the circuit  210  detects an unnecessary operation of the high-frequency wave generating circuit. Thus, no thermal influence is imposed on the heating roller  101  or any component located near the roller  101 . 
   Unlike the conventional fixing apparatus, the fixing apparatus according to the invention need not comprise a thermostat. Hence, no parts of the power line need to go around the heating roller  101 . This ensures noise reduction and operation safety. 
   Moreover, the fixing apparatus can be manufactured at a relatively low cost. This is because the current-detecting circuit  210  is configured to detect not only a control current, but also an unnecessary operation of the high-frequency wave generating circuit. 
   Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.