Touch key and induction heating cooking device employing the same

A touch key detects that an object touches the touch key. The touch key includes an electrode arranged to be touched with the object, an oscillator for outputting a high-frequency voltage and applies the high-frequency voltage to the electrode, a rectifier for rectifying and smoothing the high-frequency voltage output from the oscillator as to output a direct-current (DC) voltage, a reference voltage generator for generating a reference voltage, a subtracter for subtracting the reference voltage from the DC voltage output from the rectifier, a judging unit for detecting a voltage difference between the voltage output from the subtracter before and after changing and for determining that the object touches the electrode when the voltage difference exceeds a predetermined value. This touch key detects that an object touches the electrode with high sensitivity, and enhances the reliability of the judging unit.

THIS APPLICATION IS A U.S. NATIONAL PHASE APPLICATION OF PCT INTERNATIONAL APPLICATION NO. PCT/JP2005/018524 filed OCT. 6, 2005.

TECHNICAL FIELD

The present invention relates to a touch key and to an induction heating cooker using the touch key.

BACKGROUND OF THE INVENTION

FIG. 9 is a block diagram of conventional touch key 1001 disclosed in Japanese Patent Laid-Open Publication No. 2003-224459. Electrode2is provided on a surface of panel1made of an electrical insulator, such as glass, having a flat surface. Electrode3facing electrode2is provided on the other surface of panel1. Electrode2and3and panel1provides a capacitor. Oscillator4outputs a high-frequency voltage, and outputs the voltage to electrode3. Oscillator4has high output impedance. When finger9, a conductive object, touches electrode2, electrode3is bypassed through panel1, electrode2, and finger9to be grounded, thereby reducing the high-frequency voltage at electrode3to a voltage lower than that before finger9touches electrode2. Rectifier5rectifies and smoothes the high-frequency voltage output from oscillator4, the high-frequency voltage applied to electrode3, so as to convert the high-frequency voltage to a direct-current (DC) voltage, and outputs the DC voltage to voltage divider12. The DC voltage appearing when electrode2is not touched is higher than the voltage appearing when electrode2is touched. Voltage divider12divides the DC voltage output from rectifier5and outputs a low DC voltage having an absolute value lower than that of the DC voltage output from rectifier5to judging unit13. When the direct-current voltage input to judging unit13changes, judging unit13calculates a voltage difference between voltages before and after the change, and determines that finger9touches electrode2if the voltage difference exceeds a predetermined value. The DC voltage output from rectifier5has a high absolute value, and may accordingly break judging unit13or causes judging unit13to malfunction. In order to prevent this problem, the DC voltage output from rectifier5is divided by voltage divider13to be lowered to a predetermined level.

FIG. 10illustrates the high-frequency voltage output from oscillator4. Finger9does not touch electrode2before time T0, and continues touching electrode2after time T0. Before time T0, finger9does not touch electrode2, hence causing oscillator4to output voltage Vosc101. After time T0, finger9touches electrode2, hence causing oscillator4to output voltage Vosc102having an amplitude smaller than that of Vosc101.FIG. 11illustrates the DC voltage output from rectifier5. Voltage V10output when finger9does not touch electrode2is a DC voltage into which voltage Vosc101before time T0shown inFIG. 10is converted. Voltage V11output after time T0when finger9touches electrode2is a DC voltage into which voltage Vosc102shown inFIG. 10is converted. Thus, finger9touches electrode2, thereby producing voltage difference ΔV6between voltages V10and V11.

Judging unit13generally includes a semiconductor, such as a microprocessor, which is weak to an excessive input, and accordingly, it is important that a voltage input thereto is limited.FIG. 11shows voltage Vk, an upper limit of the voltage input to judging unit13. The voltage output from rectifier5is higher than upper limit voltage Vk, and hence cannot be input to judging unit13.

A cooker including touch key1001is operated while a cooked object is being looked at, hence being used easily.

In touch key1001, voltage difference ΔV7between voltages V12and V13is divided as well as voltages V10and V11, and thus being smaller than voltage difference ΔV6. Voltage difference ΔV6is thus small, and accordingly reduces the sensitivity of the judging of judging unit13to determine whether finger9touches electrode3or not.

SUMMARY OF THE INVENTION

A touch key detects that an object touches the touch key. The touch key includes an electrode arranged to be touched with the object, an oscillator for outputting a high-frequency voltage and applies the high-frequency voltage to the electrode, a rectifier for rectifing and smoothing the high-frequency voltage output from the oscillator as to output a direct-current (DC) voltage, a reference voltage generator for generating a reference voltage, a subtracter for subtracting the reference voltage from the DC voltage output from the rectifier, a judging unit for detecting a voltage difference between the voltage output from the subtracter before and after changing and for determining that the object touches the electrode when the voltage difference exceeds a predetermined value.

This touch key detects that an object touches the electrode with high sensitivity, and enhances the reliability of the judging unit.

REFERENCE NUMERALS

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1is a block diagram of touch key501according to Exemplary Embodiment 1 of the present invention. Electrode2is provided on a surface of panel1made of an electrical insulator, such as glass or ceramics, having a flat surface. Electrode3facing electrode2is provided on the other surface of panel1. Electrodes2and3and panel1provides a capacitor. Oscillator4outputs a high-frequency voltage, and supplies the voltage to electrode3. Oscillator4has high output impedance. When finger9, a conductive object, touches electrode2, electrode3is bypassed through panel1, electrode2, and finger9to be grounded, thereby reducing the high-frequency voltage at electrode3to a voltage lower than that before finger9touches electrode2. Rectifier5rectifies and smoothes the high-frequency voltage output from oscillator4, the high-frequency voltage supplied to electrode3, as to convert the voltage to a direct-current (DC) voltage, and outputs the DC voltage to subtracter6. The DC voltage output from rectifier5when electrode2is not touched is higher than the DC voltage output when electrode2is touched. Reference voltage generator7generates a reference voltage, a predetermined DC voltage. Subtracter6subtracts the reference voltage from the DC voltage output from rectifier5, and outputs a DC voltage having an absolute value lower than that of the DC voltage output from rectifier5to judging unit8. When the DC voltage input to judging unit8changes, judging unit8calculates a voltage difference between voltages before and after the change, and determines that finger9touches electrode2if the voltage difference exceeds a predetermined value. Power supply51supplies power to oscillator4, rectifier5, subtracter6, judging unit8, and reference voltage generator7.

FIG. 2illustrates the high-frequency voltage output from oscillator4. Finger9does not touch electrode2before time T0and continues touching electrode2after time T0. Before time T0, finger9does not touch electrode2, hence causing oscillator4to output voltage Vosc1. After time T0, finger9touches electrode2, hence causing oscillator4to output voltage Vosc2having an amplitude smaller than that of voltage Vosc1.

FIG. 3illustrates the DC voltage output from rectifier5. Voltage V1output when finger does not touch electrode2is a DC voltage into which voltage Vosc1is converted before time T0shown inFIG. 2. Voltage V2output while finger9continues touching electrode2after time T0is a DC voltage into which voltage Vosc2shown inFIG. 2is converted. Finger9touches electrode9, thus producing voltage difference ΔV1between voltages V1and V2.

FIG. 4illustrates the DC voltage output from subtracter6. Subtracter6subtracts the predetermined reference voltage output from reference voltage generator7, from voltages V1and V2output from rectifier5as to voltages V3and V4, respectively. The voltage difference between voltages V3and V4is identical to voltage difference ΔV1between voltages V1and V2. Judging unit8detects voltage difference ΔV1between voltages V3and V4, and determines that finger9touches electrode2when voltage difference ΔV1exceeds a predetermined value.

When the voltage output from rectifier5exceeds voltage V2, judging unit8detects the voltage difference between voltage V4and a voltage exceeding voltage V4, and judges that finger9moves apart from electrode2when the voltage difference exceeds a predetermined value.

Judging unit8is generally includes a semiconductor, such as a microprocessor, which is weak to an excessive input, and accordingly, it is important that a voltage input to judging unit8is limited.FIGS. 3 and 4illustrate maximum tolerable input voltage Vk which is an upper limit of a voltage input to judging unit8. Voltages V1and V2higher than voltage Vk, upon being input to judging unit8, may break judging unit8or cause judging unit8to malfunction. In this case, judging unit8has tolerable range Vr of the input voltage between 0V and maximum tolerable input voltage Vk. In touch key501according to Embodiment 1, subtracter6has voltage difference ΔV1unchanged, lowers the absolute voltage level of voltage difference ΔV1, and suppresses voltages V3and V4below tolerable input voltage Vk, thus limiting the absolute levels of voltages V3and V4within tolerable range Vr of the voltage input to judging unit8. Voltage difference ΔV1larger than voltage difference ΔV7of conventional touch key1001shown inFIG. 12is input to judging unit8, accordingly providing touch key501with high sensitivity and reliability. Further, the voltage output from subtracter6is suppressed within tolerable range Vr of the voltage input to judging unit8, accordingly increasing the reliability of judging unit8.

The touch key according to Exemplary Embodiment 2 is different from the touch key according to Embodiment 1 shown inFIG. 1in an operation of reference voltage generator7. The touch key according to Embodiment 2 is identical to the touch key according to Embodiment 1 in the other operation and components, and hence their description is omitted. A reference voltage output from reference voltage generator7is necessarily lower than voltage V2, shown inFIG. 3, output when finger9touches electrode2. The reference voltage, upon being too low, may prevent voltage V1output when finger9touches electrode2from falling below tolerable voltage Vk input to judging unit8, accordingly causing a failure or malfunction of judging unit8. According to Embodiment 2, the reference voltage output from reference voltage generator7is determined based on voltage V2output when finger9touches electrode2. For example, the reference voltage may be determined so that voltage V4, shown inFIG. 4, calculated by subtracting the reference voltage from voltage V2becomes 0V. This reference voltage minimizes voltage V3output from subtracter6when finger9does not touch electrode2as much as possible. Thus, the reference voltage is optimally determined so that the voltage output from subtracter6falls within tolerable range Vr of the voltage input to judging unit8, thus providing the touch key with high sensitivity and reliability.

The touch key according to Exemplary Embodiment 3 is different from the touch key according to Embodiment 1 shown inFIG. 1in an operation of reference voltage generator7. The touch key according to Exemplary Embodiment 3 is identical to the touch key according to Embodiment 1 in other operation and components, and hence, their description is omitted.

The amplitudes of high-frequency voltages Vosc1and Vosc2output from oscillator4may vary depending on the voltage supplied from power supply51to oscillator4. The amplitudes of output voltages Vosc1and Vosc2, shown inFIG. 4, output from oscillator4decrease, and lower voltages V1and V2shown inFIG. 3, accordingly decreasing voltage difference ΔV1and reducing sensitivity when finger9touches electrode2.

According to Embodiment 3, the reference voltage output from reference voltage generator7is determined based on the voltage supplied from the power supply to oscillator4. In the case that the amplitudes of high-frequency voltages Vosc1and Vosc2supplied from oscillator4decrease when the power supply voltage supplied to oscillator4falls, reference voltage generator7sets the reference voltage to a low level. Then, when the power supply voltage supplied to oscillator4rises, reference voltage generator7sets the reference voltage to a high level. The reference voltage supplied from reference voltage generator7is optimally determined, so that the voltage supplied from subtracter6falls in tolerable range Vr of the voltage input to judging unit8based on the power supply voltage supplied to oscillator4. This operation decreases fluctuations of voltages V3and V4due to fluctuation of the power supply voltage, thus providing the touch key having a highly sensitivity and reliability, and preventing judging unit8from failure and malfunction.

FIG. 5is a block diagram of touch key502according to Exemplary Embodiment 4. Electrode2is provided on a surface of panel1made of an electrical insulator, such as glass or ceramics, having a flat surface. Electrode3facing electrode2is provided on the other surface of panel1. Electrodes2and3and panel1provide a capacitor. Oscillator4outputs a high-frequency voltage, and supplies the voltage to electrode3. Oscillator4has high output impedance. When finger9, a conductive object, touches electrode2, electrode3is bypassed through panel1, electrode2, and finger9to be grounded, accordingly reducing the high-frequency voltage at electrode3to a voltage lower than the voltage output when finger9does not touch electrode2. Rectifier5rectifies and smoothes the high-frequency voltage supplied from oscillator4, the high-frequency voltage supplied to electrode3, to convert the voltage to a direct-current (DC) voltage, and outputs the DC voltage to differentiator10. The DC voltage output from rectifier5when electrode2is not touched is higher than the DC voltage output when electrode2is touched. Differentiator10differentiates the DC voltage output from rectifier5and outputs the differentiated DC voltage to judging unit11. Judging unit11determines that finger9touches electrode2when the voltage output from differentiator10falls. Judging unit8determines that finger9moves apart from electrode2when the voltage output from differentiator10rises.

That is, judging unit11determines that finger9touches the electrode based on the voltage output from differentiator10, so that the absolute value of the voltage input to judging unit11may not be considered.

FIGS. 6 and 7show the voltages output from rectifier5and differentiator10, respectively. Finger9does not touch electrode2before time T1. Finger9touches electrode2from time T1to time T2. Then, finger9moves apart from electrode2after time T2. Based on the high-frequency voltage output from oscillator4, rectifier5outputs voltage V5before time T1, outputs voltage V6lower than voltage V5from time T1to time T2, and outputs voltage V5after time T2. Differentiator10outputs voltage V9before time T1. When finger9touches electrode2at time T1, the voltage output from rectifier5falls from voltage V5to voltage V6, and differentiator10accordingly outputs pulse voltage V7having a falling edge. After outputting pulse voltage V7, differentiator10outputs voltage V9. When finger9moves apart from electrode2at time T2, the output voltage output from rectifier5rises from voltage V6to voltage V5, and differentiator10accordingly outputs pulse voltage V8having a rising edge. The voltage output from rectifier5is differentiated to produce the pulses falling and rising from 0V. In general, a voltage lower than a ground potential (e.g. 0V) cannot be input to judging unit11including a microprocessor. Voltage V9is a bias voltage to be added to the voltage provided by differentiating the voltage output from rectifier5at differentiator10. Voltage V9is determined so that pulse voltage V8does not exceed tolerable voltage Vk input to judging unit11, and that pulse voltage V7does not fall below 0V. Judging unit11determines that finger9touches electrode2when the voltage input thereto falls, and determines that finger9moves apart from electrode2when the voltage input thereto rises. This operation provides touch key502with high sensitivity and reliability, and prevents judging unit8from a failure and malfunction.

A touch key according to Exemplary Embodiment 5 is different from touch key502according to Embodiment 4 shown inFIG. 5in an operation of judging unit11. The touch key according to Embodiment 5 is identical to touch key502the other components, and their description is omitted. According to Embodiment 4, judging unit11determines that finger9touches electrode2when the voltage input thereto falls, and determines that finger9moves apart from electrode2when the voltage input thereto rises. According to Embodiment 5, judging unit11detects voltage difference ΔV4between voltage V7output from differentiator10and voltage V9, namely, a falling value, and detects voltage difference ΔV5between voltage V8and voltage V9, namely a rising value. Judging unit11ignores detected voltage differences ΔV4and ΔV5if the voltage differences are smaller than predetermined values. That is, according to Embodiment 5, judging unit11determines that finger9touches electrode2when the voltage input thereto falls, and when change ΔV4of the falling is larger than the predetermined value. Judging unit11determines that finger9moves apart from electrode2when the voltage input thereto rises and when change ΔV5of the rising is larger than the predetermined value. This operation provides the touch key with high sensitivity and reliability, and preventing the judging unit from failure and malfunction.

FIG. 8is a schematic diagram of induction heating cooker600according to Exemplary Embodiment 6 of the present invention. Induction heating cooker600includes cabinet601, induction heater602, and touch key501according to Embodiments 1 to 3 shown inFIGS. 1 to 4. Induction heater602includes a heating coil and an inverter for generating a high-frequency magnetic field having a frequency higher than 20 kHz. Cabinet601includes top plate601A made of ceramic. Electrode2is provided on a top surface of top plate601A at a side closer to a user. Electrode3is provided on a bottom surface of top plate601A. Top plate601A made of insulating material functions as panel1shown inFIGS. 1 and 5.

The user touches electrode2of touch key501with finger9to send an instruction, such as start/stop heating, to induction heater602, and judging unit8determines that finger9touches electrode2. This instruction is sent to induction heater602according to a judgment result of judging unit8. Induction heater602then induction-heats pot603, an object to be heated, placed on top plate601A with the high-frequency magnetic field according to the instruction.

Induction heating cooker600allows the user to activate touch key501while he/she looks at a material to be coked in pot603, the object to be heated. Particularly when electrode2is touched to activate touch key501during cooking, high sensitivity is required since electrode2or finger9is contaminated or since electrode2is lightly touched. The induction heating cooker includes the heating coil and the inverter which generate a strong high-frequency magnetic field and switching noises extremely near the touch key, the touch key is required to have high noise immunity. Touch key501has the high sensitivity and is hardly influenced by noises, hence providing reliable induction heating cooker600to be used easily, since the material to be cooked can be looked at during cooking.

Induction heating cooker600may include touch key502according to Embodiment 4 or 5 shown inFIGS. 5 to 7instead of touch key501.

Touch keys501and502according to Embodiments 1 to 8 can be used for household appliances, such as a microwave oven and other devices, as well as the induction heating cooker with the same effects.

The present invention is not limited to Embodiments 1 to 6.

INDUSTRIAL APPLICABILITY

A touch key according to the present invention has high sensitivity, reliability, and tolerance to noise, hence being useful for household appliances, such as an induction heating cooker and a microwave oven.