MICROWAVE OVEN

Abstract: A microwave oven having a heater cooking device, a damper (8) opera­ ted by the heat in the cooking chamber to control ventilation to the cooking chamber, a blower (6) for cooling the cooking chamber exterior and electric components such as a magnetron (4), and tempe­ rature sensor (7) for detecting the cooking chamber internal tempe­ rature. A control circuit actuates the blower (6) immediately when the cooking cham­ ber internal temperature at the time of starting the heater cooking operation is the same as or higher than a specified value considered high enough to actuate the damper and actuates the ventilator with a delay of a specified time when the cooking chamber internal temperature is lower than the specified value.

Description: The present invention relates to a microwave oven having a heater cooking function and equipped with a damper which is operable to control ventilation to the heating chamber and with a blower which cools the heating chamber exterior and electric components such as a magnetron. A multi-functional microwave oven having heater cooking and microwave cooking functions is equipped with a damper that shuts off ventilation to the heating chamber to minimise heat loss during heater cooking operation and which allows ventilation to the heating chamber to release steam generating from cooked food to outside the heating chamber during microwave heating operation. For this application, the industry has recently proposed a damper that opens or closes due to the shape memory effect of a shape memory alloy. Such a damper using a shape memory alloy is disclosed, for example, in U.S. Patent 4,608,474, and published British patent applications nos. 2137859 and 2123660. However, the conventional commercialised dampers using a shape memory alloy each utilise a heater installed adjacent the shape memory alloy or the heat generated in the alloy itself when it is supplied with power, as a heat source for the alloy. The conventional damper of this type involves an additional electric heating device for actuating the damper, resulting in complicated construction and therefore high cost. The present invention aims to provide a microwave oven in which a damper which is operated by the heat in the heating chamber for controlling ventilation to the heating chamber is actuated quickly and accurately when heater cooking is started. According to the present invention there is provided a microwave oven comprising: a heater and a microwave cooking means; a blower for cooling the exterior of the cooking chamber and electric components of the oven, including a magnetron of the microwave cooking means and a damper operated to control ventilation to the cooking chamber; characterised in that said damper is operated by the heat in the cooking chamber, and further characterised by: temperature sensor means for detecting the internal temperature of the cooking chamber; and control means which immediately actuates said blower if the cooking chamber internal temperature at the time of starting of a heater cooking operation is the same as or higher than a specified value considered high enough to close said damper, and which actuates said blower with a delay of a specified time if the cooking chamber internal temperature at said time is lower than said specified value. When the heating chamber temperature at the time of starting heater cooking is below the specified value (for example, about 100°C), the heating chamber is heated with the blower stopped for a specified period (for example, about 2 minutes) considered necessary to heat up the shape memory alloy spring of the damper sufficiently high enough to be closed. This permits the heating chamber temperature to rise faster. Consequently, the shape memory alloy spring of the damper is heated rapidly, allowing the damper to close quickly. Then, the heating chamber temperature rises still faster because the damper has closed. 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 structural drawing of the essential part of a microwave oven of the present invention; Fig. 2 is a flow chart for explaining the present invention; Fig. 3 is a schematic block diagram of the control device of the microwave oven of the present invention; and Fig. 4(1) and 4(2) schematically show dampers of the microwave oven of the present invention which use a shape memory alloy spring. DETAILED DESCRIPTION OF THE INVENTION An embodiment of the present invention will be described in detail with reference to the accompanying drawings. Fig. 1 shows the construction of the essential part of a microwave oven of the present invention, and Fig. 2 is a flow chart for explaining the present invention. Referring to Fig. 1, 1 is a control panel having thereon a key for selecting the desired cooking mode from various modes such as heater cooking and microwave cooking, various function keys, numeric keys for setting cooking temperature, cooking time, etc., a start key for starting cooking operation, a display, etc. 2 is a control device such as a micro processor (MPU) that controls the operation of the microwave oven. 3 is a power supply source which controls power supply to a microwave generating device 4 such as a magnetron as heating means for the microwave oven, a heater cooking device 5 and a blower 6 according to a control signal from the control device 2. 7 is temperature sensor means such as a thermistor which detects the heating chamber temperature. 8 is a damper that opens or closes due to the shape memory effect of a shape memory alloy: when the temperature in the heating chamber rises, the shape memory alloy resumes its memorized shape due to the heat, allowing the damper to close the blowhol provided for supplying ventilating air from the blower 6 to the heating chamber. The function of the control device 2 related to the present invention is now described on the basis of the flow chart of Fig. 2. It is assumed that heater cooking as gril cooking or hot-air circulating cooking by an electric heater is selected. When the start key is depressed with the heater cooking mode selected and the cooking time (T) set on the control panel 1, the control device 2 such as a micro processor (MPU) outputs a signal for actuating the heater cooking device to cook food for the preset time (T). Then, the control device 2 makes the temperature sensor means detect the heating chamber temperature (K). When the temperature (K) is the same as or higher than a specified value (For example, about 100 °C), the control device 2 sets timer means TIME (contdown timer) for actuating the blower 6 with dealy at 0. When the temperature (K) is below the specified value, the control device 2 sets the timer means TIME at a specified value (say 2 minutes) considered necessary for the heating chamber to be heated high enough to close the damper 8. Next, the control device 2 judges whether the setting of the timer means TIME is 0 or not, and outputs a signal for actuating the blower 6 whens the setting is 0. Otherwise, it waits for the timer means TIME to count down to 0 (taking about 2 minutes) before it outputs the signal for actuating the blower 6. By this time, the damper 8 has already closed the blowhole. With this state, heater cooking operation is continued for the preset cooking time (T). When the cooking time (T) has been elapsed, the control device 2 turns OFF the heater cooking device 5 and detects the heating chamber temperature (K). When it finds the temperature (K) below the specified value (for example, about 100 °C), it outputs a signal for stopping the blower 6. When the temperature (K) is not lower than the specified value, on the other hand, the control device 2 continues detecting the heating chamber temperature (K) until the temperature (K) drops below the specified value, while allowing the blower 6 to keep operating. On detecting the temperature (K) below the specified value, the control device 2 performs the blower-stopping routine. According to the present invention, as described above, the damper operated by heat of the heating chamber for controlling ventilation to the heating chamber is actuated promptly and accurately at the time of starting heater cooking so that the heating chamber temperature can be raised rapidly and efficiently. Fig. 3 is a block diagram showing the schematic construction of the control device 2, and Figs. 4(1) and 4(2) show an example of the damper used in the present invention. Referring to Fig. 3, a cooking mode selection signal input by the cooking mode selector key 1-1 on the control panel 1 is detected by a key input detector circuit 10 and stored through a control circuit 9 in a predetermined area of a memory (RAM) 11. A cooking time signal input by a cooking (heating) time setting key 1-2 is also detected by the key input detector circuit 10 and stored through the control circuit 9 in a predetermined area of the memory 11. When a cooking start signal input by a cooking start key 1-3 on the control panel 1 is detected by the key input detector circuit 10 and sent in the control circuit 9, the control circuit 9 reads the cooking mode selection signal and the cooking time signal stored in the memory 11, and sends the cooking mode selection signal to a cooking control circuit 17, and the cooking time signal to a cooking timer 14. According to the cooking mode selection signal, the cooking control circuit 17 selects an appropriate cooking device (a microwave generating device 4 or a heater cooking device 5), and supplies power from the power supply circuit 3 to the selected cooking device. The cooking timer 14 sets cooking time according to the cooking time signal and counts down the set cooking (heating) time. The count of the cooking timer 14 is checked by the control circuit 9. When the timer 14 has counted down to zero, the control circuit 9 outputs a control signal to the cooking control circuit 17 to stop supplying power to the cooking device concerned. When microwave cooking mode has been selected, the cooking control circuit 17 supplies power to the microwave generating device 4, and the control circuit 9 sends a control signal to a blower control circuit 16 to supply power from the power supply sorce 3 to the blower 6 in response to the cooking start signal input, so that the blower 6 for supplying cooling air to the cooking chamber 22 and to the microwave generating device 4 such as a magnetron is actuated simultaneously as cooking operation is started. In the microwave cooking mode, the blower 6 is kept operated until the cooking timer 14 has counted down to zero. When heater cooking mode has been selected, the control circuit 9 sends a control signal, in response to the input by the cooking start key, to the heater cooking device 5 to operate it for the preset cooking time. Also in response to the input by the cooking start key, the control circuit 9 controls the temperature detector circuit 13 so that the temperature sensor means 7 such as the thermistor detects the temperature in the cooking chamber. The cooking chamber temperature detected is sent to the control circuit 9 for comparison with a reference temperature (H) stored in advance in a memory (ROM) 12. When the cooking chamber temperature is the same as or higher than the reference temperature (H), the control circuit 9 sends a control signal to the blower control circuit 16 to actuate the blower 6. When the cooking chamber temperature is the same as or lower than the reference temperature (H), on the other hand, the control circuit 9 reads a specified time (I) stored in advance in the ROM 12 and sets it in a timer 15 for actuating the blower 6 with a delay so that the timer 15 counts down for the specified time (I). The blower 6 does not operate while the timer 15 is counting down. When the control circuit 9 understands the timer 15 has counted down to zero, the blower control circuit 16 actuates the blower 6. Namely, the blower 6 is fed with power from the power supply circuit 3. When the control circuit 9 understands that the cooking timer 14 has counted the preset cooking time, the cooking control circuit 17 works to shut off power supply to the heater cooking device. At this time, the control circuit 9 outputs a control signal to the temperature detector circuit 13 so that the temperature sensor means 7 detects the cooking chamber internal temperature. The control circuit 9 compares the detected temperature with the reference temperature (H) stored in the ROM 12. When the detected temperature is lower than the reference temperature (H), the blower control circuit 16 works to stop the blower 6, whereas if it is the same as or higher than the reference temperature (H), the blower 6 is allowed to continue operating. In the latter case, the cooking chamber internal temperature is detected and compared with the reference temperature (H) periodically, and when it is judged to be lower than the reference temperature (H), the blower control circuit 16 stops the blower 6. Here, the reference temperature (H) stored in the ROM 12 is the temperature at which the shape memory alloy spring used in the damper shown in Figs. 4(1) and 4(2) changes its shape due to the shape memory effect. (It is, for instance, at about 100°C.) The specified time (I) stored in the ROM 12 is the time required for the cooking chamber internal temperature to reach the reference temperature (H). (It is, for instance, about 2 minutes.) Fig. 4(1) shows the state of the damper in the microwave cooking mode in which a damper lid 18 need not be closed. Cooling air supplied by the blower 6 is led through the damper 8 and the blowhole 21 into the cooking chamber 22. At a normal temperature, the shape memory alloy spring 19 has lower elasticity than the bias spring 20. During microwave cooking, since the temperature in the cooking chamber does not rise, the shape memory alloy spring 19 does not change in shape. Fig. 4(2) shows the state of the damper in the heater cooking mode in which the damper lid 18 must be closed. When the cooking chamber internal temperature rises to or above the specified reference temperature, the shape memory alloy spring 19 changes in shape so that it provides higher elasticity than the bias spring 20, causing the damper lid 18 to be closed as shown in Fig. 4(2). The blower 6 is actuated after the damper lid 18 is closed. Therefore, cooling air is not led into the cooking chamber but cools electrical components outside the cooking chamber. In the above embodiment, the specified time (I) stored in the ROM 12 is constant independent of the cooking chamber internal temperature. Alternatively, a time value calculated on the basis of the initial temperature in the cooking chamber may be set in the timer 15, or an appropriate specified time (I) may be selected depending upon the measurement of the cooking chamber internal temperature from among a plurality of specified times stored in advance for different cooking chamber internal temperatures in the ROM 12. Furthermore, in the above embodiment, the damper lid is operated by means of the shape memory alloy spring. It may be operated by means of bimetal. As described above, in the present invention, the blower is driven in response to the detection of the temperature at which the shape of the shape memory alloy spring or the bimetal used in the damper is changed. Therefore, the damper is actuated quickly and accurately by the heat of the heating chamber when the heater cooking is started. While only certain embodiments of the present invention have been described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the present invention as claimed.

Claims: 1. A microwave oven comprising: a heater (5) and a microwave cooking means (4); a blower (6) for cooling the exterior of the cooking chamber (22) and electric components of the oven, including a magnetron (4) of the microwave cooking means; and a damper (8) operated to control ventilation to the cooking chamber; characterised in that said damper (8) is operated by the heat in the cooking chamber, and further characterised by: temperature sensor means (7) for detecting the internal temperature of the cooking chamber (22); and control means (2) which immediately actuates said blower (6) if the cooking chamber internal temperature at the time of starting of a heater cooking operation is the same as or higher than a specified value considered high enough to close said damper, and which actuates said blower (6) with a delay of a specified time if the cooking chamber internal temperature at said time is lower than said specified value. 2. A microwave oven according to claim 1, wherein said control means is adapted so that if said delay is applied, said specified time is constant, independent of the cooking chamber internal temperature detected by said temperature sensor means. 3. A microwave oven according to claim 1, wherein said control means (2) is adapted so that if said delay is applied, said specified time is predetermined in accordance with the cooking chamber internal temperature detected by said temperature sensor means. 4. A microwave oven according to any preceding claim wherein said damper (8) is operated by means of a shape memory alloy spring. 5. A microwave oven according to any of claims 1 to 3 wherein said damper (8) is operated by means of a bimetal element. 6. A microwave oven according to any preceding claim wherein said control means (2) is operable on termination of a said heater cooking operation to turn off said blower when the cooking chamber internal temperature detected by said sensor means falls below said specified value.

Inventors: HIRATA,KENGO, ISHIFURO,KATHUMI
Applicants: SHARP KABUSHIKI KAISHA
Application year: 1987
Main classification: H05B6
IPC codes: H05B6, F24C7