Fixing apparatus

A coil for generating a high-frequency magnetic field is provided in a heating roller. The coil is connected to a capacitor, constituting a resonant circuit. The resonant circuit is incorporated in a high-frequency wave generating circuit. A current-detecting circuit detects the current supplied to the high-frequency wave generating circuit. The supply of the current to the high-frequency wave generating circuit is controlled in accordance with the current detected by the current-detecting circuit and the logic level of an operation-on signal.

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.

DETAILED DESCRIPTION OF THE INVENTION

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 (unit95, described later) forms, on a paper sheet, a toner image corresponding to the image read by the scanning unit. The fixing apparatus (apparatus100, 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. 1depicts the fixing apparatus100. As shown inFIG. 1, the fixing apparatus100comprises a heating roller101and a pressing roller102. The heating roller101is located above the copy-sheet path. The pressing roller102lies below the copy-sheet path and contacts the heating roller101, pressed onto the roller101by means of a pressing mechanism (not shown). The contacting parts of the rollers101and102form a nip. The nip has a prescribed length.

The heating roller101comprises 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 roller101can be rotated clockwise in FIG.1. The pressing roller102can be rotated counter-clockwise. A copy sheet S may pass through the nip between the heating roller101and the pressing roller102. While passing through the nip, the sheet S receives heat from the heating roller101. The toner image T on the sheet S is thereby fixed.

A sheet-peeling claw103, a cleaning member104, and a release-agent applying roller105are arranged around the heating roller101. The sheet-peeling claw103is designed to peel a copy sheet S from the heating roller101. The cleaning member104is configured to remove residual toner, paper dust and the like from the heating roller101. The release-agent applying roller105is provided to apply a release agent to the outer circumferential surface of the heating roller101.

The heating roller101incorporates a coil111that performs induction heating. The coil111is wound and held around a core112. It is designed to generate a high-frequency magnetic field to achieve induction heating. When the coil111generates a high-frequency magnetic field, an eddy current is induced in the heating roller101. The roller101generates Joule heat from the eddy current.

The heating roller101that is used as a heating member can be replaced by a belt made of electrically conductive material. The coil111may be arranged outside the heating roller101, not in the roller101as shown in FIG.1.

FIG. 2shows the control circuit incorporated in the electronic copier according to the invention. AsFIG. 2shows, the control circuit comprises a main CPU50, a scan CPU70, a control-panel CPU80, and a print CPU90. The CPUs70,80and90are connected to the main CPU50.

A ROM91, a RAM92, a print engine93, a sheet-feeding unit94, a processing unit95, and the fixing apparatus100are connected to the print CPU90. The ROM91stores control programs. The RAM92is provided to store data.

FIG. 3depicts the electric circuit of the fixing apparatus100. AsFIG. 3shows, a switch201having normally closed contacts201aand201bconnects a variable resistor202and rectifying circuit203to the commercially available power supply200. The output of the rectifying circuit203is connected to a smoothing capacitor203. The smoothing capacitor204is connected to a resonant circuit that comprises the above-mentioned coil111and a capacitor205. A switching element (transistor)206is provided on the current path to the resonant circuit to excite the resonant circuit. The switching element206is turned on or off by a drive signal supplied from a drive circuit207. When the element206is turned on, the resonant circuit is excited, inducing a high-frequency current in the coil111. As a result, the coil111generates a high-frequency magnetic field.

The variable resistor202, rectifying circuit203, smoothing capacitor204, capacitor205and switching element206constitute a high-frequency wave generating circuit that supplies a high-frequency current to the coil111. A current-detecting circuit210is provided on the current path (power-supply line) extending between the commercially available power supply200and the high-frequency wave generating circuit.

The current-detecting circuit210comprises a voltage-lowering transformer211, rectifying circuit212and a parallel circuit. The transformer211has its primary winding connected to the current path to the high-frequency wave generating circuit. The rectifying circuit212is connected to the secondary winding of the transformer211. The parallel circuit comprises a resistor213and a smoothing capacitor214, both connected to the output of the rectifying circuit212. The current-detecting circuit210outputs a DC voltage that corresponds to the current input to the high-frequency wave generating circuit. The DC voltage is applied to the IH CPU215, which is a control unit.

The IH CPU215receives an output of the coil111from the current detected by the current-detecting circuit210while the operation-on signal supplied from the print CPU90remains at logic “1” level. The IH CPU215controls the drive circuit207, which drives the high-frequency wave generating circuit so that the output of the coil111may remain at a predetermined value.

A temperature sensor301is mounted on the outer circumferential surface of the heating roller101. The temperature sensor301receives a DC voltage Vd via resistors302and303. Hence, the voltage generated across the resistor303rises as the temperature of the heating roller101rises, decreasing the resistance of the temperature sensor301.

The voltage generated across the resistor303is applied to the print CPU90as a signal that represents the temperature the sensor301has detected.

The voltage generated across the resistor302is applied to the negative (−) input terminal of a comparator304, too. The DC voltage Vd is applied to a series circuit comprising resistors305and306. The voltage generated across the resistor306is applied as reference voltage to the positive (+) input terminal of the comparator304. The output of the comparator304is at logic “1” level as long as the voltage generated across the resistor303remains lower than the reference voltage. It falls to logic “0” level when the voltage generated across the resistor303becomes equal to or higher than the reference voltage (or when the temperature of the heating roller101rises to excess). The output of the comparator304is input to one input terminal of an AND circuit307.

The other input terminal of the AND circuit307receives the operation-on signal (at logic “1” level) from the print CPU90. The operation-on signal supplied from the CPU90to the AND circuit307is at logic “1” level or logic “0” level in order to maintain the temperature of the heating roller101(i.e., the temperature detected by the sensor301) at a preset temperature. As shown in the flowchart ofFIG. 4, it is determined in Step S101whether the temperature detected by the sensor301is lower than the preset temperature. If YES, the operation-on signal is set at logic “1” level in Step S102. If NO in Step S101, the operation-on signal is set at logic “0” level in Step S103.

The AND circuit307outputs the operation-on signal at logic “1” level when the output of the comparator304is at logic “1” level. The operation-on signal at logic “1” level is supplied to the IH CPU215.

In the meantime, the output voltage of the current-detecting circuit210is input to the positive (+) input terminal of a comparator401. The DC voltage Vd is applied to a series circuit that comprises resistors402and403. A voltage generated across the resistor403is applied as reference voltage to the negative (−) input terminal of the comparator401. The output of the comparator401is at logic “0” level as long as the output voltage of the current-detecting circuit210remains lower than the reference voltage. When the output voltage of the circuit210becomes equal to or higher than the reference voltage, the output of the comparator401rises to logic “1” level. The output of the comparator401is supplied to one input terminal of an AND circuit404. The other input terminal of the AND circuit404receives an operation-on signal (at logic “1” level) supplied from the AND circuit307through an inverter405.

The comparator401, resistors402and403, AND circuit404, inverter405and switch201constitute a protection circuit400. If the AND circuit307outputs no operation-on signal and the output of the inverter405therefore remains at logic “1” level, the output of the AND circuit404rises to logic “1” level when the current-detecting circuit210detects 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 circuit404, which is at logic “1” level, opens the normally closed contacts201aand201bof the switch201. 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 CPU90outputs an operation-on signal at logic “1” level, which is supplied to the IH CPU215via the AND circuit307. As long as the operation-on signal remains at logic “1” level, the IH CPU215keeps driving the drive circuit207. Thus, the drive circuit207drives the high-frequency wave generating circuit, which generates power. The power excites the resonant circuit that comprises the coil111and capacitor205. That is, a high-frequency current flows in the coil111, which generates a high-frequency magnetic field. The high-frequency magnetic field induces an eddy current in the heating roller101. The heating roller101generates Joule heat from the eddy current.

When the temperature sensor301detects that the temperature of the heating roller101becomes too high, the output of the comparator304acquires logic “1” level. At the same time the AND circuit307blocks the operation-on signal input from the print CPU90. Therefore, the operation-on signal is not supplied to the IH CPU215. The IH CPU215stops controlling the high-frequency wave generating circuit. The heating roller101becomes no longer heated too much.

The current-detecting circuit210detects the input current to the high-frequency wave generating circuit while the high-frequency wave generating circuit is operating. The circuit210generates a signal that represents the input current detected. The signal is supplied to the IH CPU215. The IH CPU215determines the output of the coil111from the current that the circuit210has detected. The IH CPU215controls the high-frequency wave generating circuit to cause the output of the coil111to have a prescribed value.

The print CPU90monitors the output signal of the detector301, which represents the temperature of the heating roller102. Based on the magnitude of the output signal of the detector301, the print CPU90keeps or stops outputting the operation-on signal to maintain the temperature of the heating roller101at a preset value. When the CPU90stops outputting the operation-on signal, the high-frequency wave generating circuit stops operating.

Nonetheless, the IH CPU215may malfunction to keep driving the high-frequency wave generating circuit, even if the print circuit90has 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 circuit210detects this current. The protection circuit400processes the output of the circuit210and the operation-on at logic “1” level, generating an output at logic “1” level. The output of the protection circuit400, which is at logic “1” level, opens the normally-closed contacts201aand201b. 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 circuit210detects 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 roller101is heated to excess, when the circuit210detects an unnecessary operation of the high-frequency wave generating circuit. Thus, no thermal influence is imposed on the heating roller101or any component located near the roller101.

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 roller101. 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 circuit210is configured to detect not only a control current, but also an unnecessary operation of the high-frequency wave generating circuit.