Defrosting using a microwave oven

Abstract: A microwave oven uses the initial temperature, sensed with an IR sensor (106), to determine a defrost target temperature and then applies microwave energy to a frozen food item (F) until the target temperature is reached. The level of microwave energy may be reduced as the defrosting process progresses.

Description: The present invention relates to a method of defrosting using a microwave oven, comprising applying microwave energy to a food item, preferably by means of a magnetron, while monitoring the surface temperature of the food item by sensing means, preferably infrared sensing means and to a microwave oven comprising a food receiving area, means for applying microwave energy to items in said area, sensing means for detecting the surface temperature of a food item in said area and control means for controlling defrosting of food items in said area. A conventional microwave oven is shown in Figure 1. Referring to Figure 1, the microwave oven includes a body 10 and cooking and electrical component chambers 12, 14 within the body 10. Food to be cooked is placed in the cooking chamber 12. A door 20 is provided for opening and closing the cooking chamber 12. A turntable 16 is located at the bottom of the cooking chamber 12. The electrical component chamber 14 includes various devices for generating and emitting microwaves to the cooking chamber 12, including a magnetron 17, a high-voltage transformer 18, a waveguide (not shown) and a cooking fan 19. A control panel 30 is mounted at the front of the electrical component chamber 14. The control panel 30 enables a user to input instructions into the oven. Food is cooked in accordance with the instruction input using the control panel 30 by a control part (not shown) which is formed at the back of the operation panel 30. When the devices in the electrical component chamber 14 are operated, the microwaves generated by the magnetron 17 are guided through the waveguide into the cooking chamber 12. The microwaves either irradiate the food directly or after being reflected from the walls of the cooking chamber 12. In addition to cooking, microwave ovens can be used for defrosting frozen food and for warming drinks. A conventional method for defrosting food will now be described with reference to Figure 2. Referring to Figure 2, the frozen food is initially weighed (Step S1) using a weight sensor in the oven. Alternative prior art methods require the user to input the weight using the control panel 30. After weighing of the food, a defrosting time is set in dependence on the measured weight of the food (Step S2). Next, the magnetron 17, or other microwave generator, is operated for the defrosting time (Step S3). When the defrosting time has elapsed (Step S4), the magnetron 17 is stopped and the defrosting process is complete (Step S5). The reader is also referred to US-A-4401884 for additional background information. A method of defrosting using a microwave oven, according to the present invention, is characterised by initially setting a defrost completion value in dependence on the initial output of the sensing means, and the application of microwave energy being performed until the output of the sensing means, preferably detected regularly, reaches the defrost completion value. Thus the problem of the conventional defrosting method using a microwave oven, in which no allowance is made for different initial temperatures of the food to be defrosted, is overcome. Preferably, the initial value is set on the basis of the lowest value obtained while rotating the food item on a turntable. Preferably, the magnetron is driven such that the difference between the initial value and the defrost completion value is divided into at least two divisions, and the power level of the magnetron is varied in accordance with the respective divisions. More preferably, the power level of the magnetron of the respective divisions, is decreased from the value which is closer to the initial value to the value which is closer to the completion value. Preferably, the output value of the sensing means is detected regularly while rotating the food item on a turntable, and the current surface temperature is determined using the lowest output value of the sensing means from a number of output values. Preferably, the defrost completion value is set independently of the weight of the food item. Thus, the problem of the conventional defrosting method in which, usually, the user places the food to be defrosted on a plate or in a bowl to catch the water that emerges during defrosting and the weight of the plate or bowl is included in the measured weight, making the calculated defrosting time incorrect, is overcome. More preferably, the surface temperature of the food item is determined by scanning a food receiving area containing the food item with an infrared sensor and selecting the lowest temperature detected. Still more preferably, the food receiving area is the upper surface of a turntable and the infrared sensor has a fixed field of view. Preferably, the microwave energy is applied at reducing levels until the defrost completion value is reached. Preferably, the difference between the initial value and the defrost completion value is divided into a plurality of temperature bands and the microwave energy is applied at different respective levels in dependence on the band into which the detected surface temperature of the food item falls during the application of microwave energy thereto. A microwave oven, according to the present invention, is characterised in that the control means is responsive to the output of said sensing means to set a defrost completion surface temperature in dependence on an initial surface temperature and is configured to apply microwave energy, while monitoring the food item's surface temperature, until said target surface temperature is reached. Preferably, the sensing means is an infrared sensor means. Preferably, the control means is configured to set said defrost completion temperature independently of the weight of the food item. Preferably, scanning means are included for scanning said area with the infrared sensor means, wherein the control means selects the lowest temperature detected during scanning by the scanning means as the surface temperature of the food item. More preferably, the scanning means comprises a turntable having said area on its upper surface and the infrared sensor has a fixed field of view. Preferably, the control means controls the means for applying microwave energy such that the microwave energy is applied at reducing levels until the defrost completion temperature is reached. More preferably, the control means is configured to divide the difference between the initial surface temperature and the defrost completion temperature into a plurality of temperature bands and control the means for applying microwave energy to apply microwave energy at different respective levels in dependence on the band into which the detected surface temperature of the food item falls during the application of microwave energy thereto. An embodiment of the present invention will now be described, by way of example, with reference to Figures 3, 4 and 5 of the accompanying drawings, in which: - Figure 1 is a perspective view of a conventional microwave oven; Figure 2 is a flow chart illustrating a conventional defrosting method; Figure 3 is a flow chart illustrating a defrosting method of a microwave oven according to the present invention; Figure 4 is a sectional view of a microwave oven according to the present invention; and Figure 5 is a plan view of the turntable of the oven of Figure 4. Referring to Figure 4, an infrared sensor 106 is located at an upper front position relative to a cooking chamber 102 of a microwave oven, in order to detect the surface temperature of the food F placed within a detection spot Sp (See Figure 5) occupying a predetermined area of a turntable 104. A driving motor 108 for rotating the turntable 104 is located under the cooking chamber 102 and a door 110 is provided for opening and closing the cooking chamber 102. A defrosting method employing the microwave oven of Figure 4 will now be described. Referring to Figure 3, an initial value Ts detected by the infrared sensor 106 is established (Step S11). The initial value Ts obtained in S11 corresponds to the initial surface temperature of the frozen food F. The infrared sensor 106 outputs a voltage signal corresponding to the average temperature of the area occupied by the detection spot Sp. Accordingly, the voltage signal varies in dependence on the size of the frozen food F and the position of the frozen food F with respect to the turntable 104. More specifically, when the frozen food F is small and off-centre with respect to the turntable 104, as shown in Figure 5, the food F and part of the upper surface of the turntable 104 are simultaneously occupied by the detection spots Sp. In such a situation, the output value of the infrared sensor 106 corresponds to the average temperature of the surface temperature of the food F and the temperature of the upper side of the turntable 104. The problem is that the surface temperature of the food F (-20°C to -5°C in general) and the temperature of the upper side of the turntable 104 (at least room temperature) have a wide gap between them. Accordingly, the output of the infrared sensor 106 does not accurately reflect the actual surface temperature of the food F. However, the larger the area of the detection spot Sp occupied by the food F, the more accurate is the output value of the infrared sensor 106. The detection spots Sp of the infrared sensor 106 is made to occupy a certain area of the upper surface of the turntable 104, and the output value of the infrared sensor 106 is detected for a predetermined time period while the turntable 104, e.g. twice, and detected on a regular basis such as every second or every two seconds. Then the lowest output value of the infrared sensor 106 is determined to be the correct initial value for the infrared sensor 106. When the detection spot Sp is made to occupy a certain predetermined area of the upper surface of the turntable 104, the detection spot Sp is scanned circularly across the upper surface of the turntable 104 as it is rotated. Accordingly, as the detection spot Sp scans the turntable 104, the food F and the surface of the turntable 104 are sensed by the detection spots Sp in different proportions. The output value of the infrared sensor 106, which is obtained when the largest area of food F is covered by the detection spot Sp, is closest to the actual initial surface temperature of the food. Further, since the temperature of the upper surface of the turntable 104 is higher than the surface temperature of the food F, the average temperature becomes lower when a greater as the area of food increases. As the average temperature becomes lower, the output value of the infrared sensor 106 becomes lower. Accordingly, the lowest value of the output values of the infrared sensor 106 is the closest value with respect to the actual initial surface temperature of the food F. After determining the initial value Ts of the infrared sensor 106, the completion value Te is determined to determine the time when the defrosting process is completed (Step S12). The completion values Te are pre-stored in the memory, which is employed in the control part for controlling the operation of the microwave oven. Table 1 below shows the respective completion values Te varying in in dependence on the initial values Ts established using the infrared sensor 106. Initial output value Ts of infrared sensor (arbirary units) 59-60 61 62 63-64 65-66 67-68 Completion value Te of infrared sensor (arbitrary units) 69 70 71 72 73 74 Power rate for divisions D1 (40%) 59, 60-62 61-63 62-64 63, 64-65 65, 66-67 67, 68-69 D2 (20%) 63-66 64-66 65-67 66-68 68-69 70-71 D3 (10%) 66-68 67-69 68-70 69-71 70-72 72-73 As shown in Table 1, the initial value Ts of the infrared sensor 106 ranges from 59 to 68, corresponding to a surface temperature of the food F approximately in the range -20°C to -2°C. The corresponding completion value Te ranges from 69 to 74, corresponding to the defrost completion temperature, approximately in the range 0°C to 10°C. As described above, the completion value Te varies depending on the initial values Ts. This is to prevent the incomplete defrosting of food F due to too short a defrosting time. If the completion value Te is uniformly set, the defrosting time may be shortened when the initial value Ts has a narrow gap with the completion value Te. The output value of the infrared sensor 106 corresponding to the temperature of the food F may be varied depending on the types of the infrared sensor 106. After the initial value Ts of the infrared sensor 106 and the completion value Te have been determined, the magnetron is driven while the current value (Tc) of the infrared sensor 106 output, which corresponds to the surface temperature of the food F, is detected on a regular basis, until the Tc reaches the completion value Te. As a result of experiments by the inventor, it has been found that food F defrosts more efficiently when the defrosting process is started with a stronger power of the magnetron and ends with less power. First, the gap between the initial value Ts and the completion value Te is divided into three divisions, D1, D2, and D3. Like the completion values Te, the ranges of the three divisions D1, D2, and D3 are pre-stored in the memory of the controlling part. Accordingly, when the initial value Ts is detected, the ranges of the three divisions D1, D2, and D3 are determined by reading those that correspond to the initial value Ts from the memory of the control part. According to the above Table 1, when the initial value Ts is 60, the completion value Te is 69, and the ranges of the three divisions D1, D2, and D3 are 60-62, 63-65, and 66-69, respectively. After the ranges of the divisions D1, D2, and D3 are obtained in accordance with the initial value Ts, the current value Tc is detected (Step S14). The current value Tc is detected by the same method that is employed for detecting the initial value Ts in S11. However, a difference lies in that the current value Tc is preferably obtained by detecting the output value of the infrared sensor 106 on a predetermined time basis during the time in which the turntable 104 is rotated once, while the initial value Ts is preferably obtained by detecting the output value of the infrared sensor 106 for a predetermined time period. After the current value Tc has been detected, the current value Tc is compared with the completion value Te. If the current value Tc is less than the completion value Te, it is determined to which division of the three divisions D1, D2, and D3 the current value Tc falls (Step S16). If it is determined that the current value Tc falls into the division D1, the power level of the magnetron is set to 40% of maximum (Step S17). If it is determined that the current value Tc falls into the division D2, or D3, the power level of the magnetron is set to 20%, or 10%, respectively (Steps S18 and S19). The power levels of the magnetron are averages and expressed as percentages to indicate the time when the magnetron is actually driven in a predetermined time period. More specifically, the power level 40%, for example, means that the magnetron is driven periodically for 40% of the unit time period and not driven for 60% of the unit time period. As defrosting is performed, since the current value Tc of the infrared sensor 106 varies from the initial value Ts to the completion value Te, the current value Tc would pass through the three divisions D1, D2, and D3, sequentially. Accordingly, the power rate of the magnetron is adjusted from 40% in the division D1, to 20% in the division D2, and to 10% in the division D3, sequentially. Then the process returns to S14, from where the steps of S14, S15, S16, and S17 (or S18 and S19) are repeatedly performed until the current value Tc reaches the completion value Te. If the current value Tc, which is compared with the completion value Te in S15, is equal to or greater than the completion value Te, it is determined that the defrosting process is completed, so that the process exits the loop and the operation for defrosting process such as driving the magnetron, etc is stopped. According to the preferred embodiment, although the power of the magnetron is set at 40%, 20%, and 10% for the three divisions D1, D2, and D3, respectively, it is not limited to this case only, but can be varied only if the power rate of the magnetron is decreased as the current value Tc gets closer to the completion value Te from the initial value Ts. As described above, according to the present invention, since the defrosting method controls the defrosting process through the output value of the infrared sensor 106, which corresponds to the surface temperature of the food F, the accurate defrost can be performed regardless of the frozen degree of the food F and presence/absence of the receptacle for food F.

Claims: A'method of defrosting using a microwave oven, comprising applying microwave energy to a food item (F), preferably by means of a magnetron (17), while monitoring the surface temperature of the food item (F) by sensing means, preferably infrared sensing means (106), characterised by initially setting a defrost completion value in dependence on the initial output of the sensing means, and the application of microwave energy being performed until the output of the sensing means, preferably detected regularly, reaches the defrost completion value. A method according to claim 1, wherein the initial value is set on the basis of the lowest value obtained while rotating the food item on a turntable (104). A method according to claim 1 or 2, wherein the magnetron (17) is driven such that the difference between the initial value and the defrost completion value is divided into at least two divisions, and the power level of the magnetron (17) is varied in accordance with the respective divisions. A method according to claim 3, wherein the power level of the magnetron (17) of the respective divisions, is decreased from the value which is closer to the initial value to the value which is closer to the completion value. A method according to any preceding claim, wherein the output value of the sensing means is detected regularly while rotating the food item on a turntable (104), and the current surface temperature is determined using the lowest output value of the sensing means from a number of output values. A method according to claim 1, wherein the defrost completion value is set independently of the weight of the food item (F). A method according to claim 6, wherein the surface temperature of the food item (F) is determined by scanning a food receiving area containing the food item (F) with an infrared sensor (106) and selecting the lowest temperature detected. A method according to claim 7, wherein the food receiving area is the upper surface of a turntable (104) and the infrared sensor (106) has a fixed field of view (Sp). A method according to claim 6, 7 or 8, wherein the microwave energy is applied at reducing levels until the defrost completion value is reached. A method according to any preceding claim, wherein the difference between the initial value and the defrost completion value is divided into a plurality of temperature bands and the microwave energy is applied at different respective levels in dependence on the band into which the detected surface temperature of the food item (F) falls during the application of microwave energy thereto. A microwave oven comprising a food receiving area, means (17) for applying microwave energy to items in said area, sensing means for detecting the surface temperature of a food item (F) in said area and control means (30) for controlling defrosting of food items (F) in said area, characterised in that the control means (30) is responsive to the output of said sensing means to set a defrost completion surface temperature in dependence on an initial surface temperature and is configured to apply microwave energy, while monitoring the food item's surface temperature, until said target surface temperature is reached. A microwave oven according to claim 11, wherein the sensing means is an infrared sensor means (106). A microwave oven according to claim 11 or 12, wherein the control means (30) is configured to set said defrost completion temperature independently of the weight of the food item (F). A microwave oven according to claim 12 or 13, including scanning means for scanning said area with the infrared sensor means (106), wherein the control means (30) selects the lowest temperature detected during scanning by the scanning means as the surface temperature of the food item (F). A microwave oven according to claim 14, wherein the scanning means comprises a turntable (104) having said area on its upper surface and the infrared sensor (106) has a fixed field of view (Sp). A microwave oven according to claim 11, 12, 13, 14 or 15, wherein the control means (30) controls the means (17) for applying microwave energy such that the microwave energy is applied at reducing levels until the defrost completion temperature is reached. A microwave oven according to claim 16, wherein the control means (30) is configured to divide the difference between the initial surface temperature and the defrost completion temperature into a plurality of temperature bands and control the means (17) for applying microwave energy to apply microwave energy at different respective levels in dependence on the band into which the detected surface temperature of the food item (F) falls during the application of microwave energy thereto.

Inventors: KIM WON-HO, KIM, WON-HO
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1999
Main classification: H05B6
IPC codes: F24C7, H05B6