Microwave oven with heating element protection

Abstract: An apparatus for controlling the operation of a heating element (20) of a microwave oven detects the contact between the heating element (20) and cooking food being raised on a tray within the microwave oven. The oven comprises: a tray (12) on which food to be cooked is loaded; elevating means (41, 44) for elevating and lowering the tray (12); position sensing means (50a, 50b) for sensing the height of food loaded on the tray (12); and control means (100) for comparing the height of the food sensed by said position sensing means with a reference height and then controlling the operation of the heating element (20) and the elevating means (41, 43) based on the result obtained by said position sensing means (501, 50b).

Description: The present invention relates to a microwave oven comprising a cooking chamber, a elevatable tray for supporting food to be cooked, a heating element located towards the top of the chamber and control means. A conventional microwave oven will now be described. Referring to Figure 3, which is a cross-sectional view from the front of a conventional microwave oven, a microwave oven includes a metallic cabinet 10, a cooking chamber 11, a magnetron (not shown) which radiates microwaves into the cooking chamber 11, a high-voltage transformer 13 which supplies power to the magnetron, a heating element 17 mounted on the upper portion of the cooking chamber 11 for supplementary cooking, a an elevatable rotating tray 12, onto which food is placed for cooking, at the bottom of the cooking chamber 11, a shaft 31 having its upper end connected to the bottom of the tray 12 and its lower end extending below the cooking chamber 11, an elevation guide member 34 positioned under the shaft 31 for elevating the shaft 31, an elevation motor 33 for laterally reciprocating the elevation guide member 34, a rotation motor 32 which rotates the shaft 31 via a gear 32a engaged, and a weight sensing unit 35 provided under the elevation guide member 34 for weighing food on the tray 12. The operation of the above-described conventional microwave oven will now be described with reference to Figure 3. A microwave oven cooks food by applying microwaves at approximately 2 450 MHz, generated by its magnetron, to food contained in its metallic case. When microwaves are applied to food, the food's polar molecular align with the electric field components of the microwave radiation. Since the electric field is, however, alternating 2,450,000,000 times per second, i.e. the frequency of the microwaves, heat is generated by collisions between the molecules, resulting in the heating of the food. Recently, an electric heating element has been incorporated into the cooking chamber of the microwave oven so that convection heat and radiant heat generated by the electric heating element are utilized to supplement the cooking process of the microwave oven. As shown in Figure 3, a conventional microwave oven can be divided into a cooking chamber 11 defined by the metallic cabinet 10 in which food is cooked, and an electrical component compartment 14 which accommodates the oven's electrical components. The magnetron provided in the electrical component compartment 14 radiates microwaves into the cooking chamber 11 to cook food therein, and the high voltage transformer 13 powers the magnetron. Further, the heating element 17 which supplements the cooking process with radiational heat is mounted to an upper portion of the cooking chamber 11. The tray 12 on the inner bottom of the cooking chamber 11 is loaded with the food to be cooked, and rotates in a horizontal plane or moves up and down in the cooking chamber 11. The upper end of the shaft 31 is connected with the bottom of the tray 12, and the lower end of the shaft 31 extends below the cooking chamber 11. The elevation guide member 34 has an inclined surface and is installed under the shaft 31 to elevate it. The elevation motor 33 reciprocates the elevation guide member 34 laterally and the rotation motor 32 rotates the shaft 31 via a gear 32a between the shaft 31 and the motor 32. Normally, the heating element 17 is not used, so the tray 12 remains in its lowered position. However, if the user selects a grill mode, a barbecue mode or pizza-baking mode, in which the operation of the heating element 17 is needed, by pressing buttons on a control panel (not shown), the heater 17 is turned on and the tray 12 rotated and also elevated to a predetermined height. Further, if the user selects one of the above modes, he may also place a grill on the tray 12, and then start the cooking process after placing food onto the grill. In the cooking process of the conventional microwave oven, if the tray is elevated after being loaded with a lot of food, the food could be burned by contact with the heating element. This can also deform the food and contaminate the heater which would then require troublesome cleaning before reuse. A microwave oven including grill means which can be moved in an optimum position relative to the food is disclosed in GB-A-2 106 374. It is an aim of the present invention to overcome the afore-mentioned problems. An oven according to the present invention is characterised by sensing means for sensing the presence of food in contact with or in close proximity to the heating element, the control means being responsive to the sensing means to terminate a cooking operation of the oven when food in contact with or close proximity to the heating element. Furthermore according to the present invention an apparatus for controlling the operation of a heater for a microwave oven which uses microwaves generated by a magnetron and heat generated by a heater to cook food comprises: a tray on which food to be cooked is loaded; elevating means for elevating and lowering said tray; position sensing means for determining the height of the food loaded on said tray; and control means for comparing the height of the food sensed by said position sensing means with a reference height and then controlling said a heater and said elevating means based on the result obtained by said position sensing means. Preferably, the sensing means comprises a photoemitter and a photosensor. More preferably, the photoemitter and the photosensor are mounted opposite each other across the chamber. Embodiments of the present invention will now be described, by way of example, with reference to Figures 1 and 2 of the accompanying drawings, in which:- Figure 1 is a cross-sectional view from the front of a microwave oven according to the present invention; Figure 2 is a block diagram of a control circuit for the microwave oven of Figure 1; and Figure 3 is a cross-sectional view front the front of a prior art microwave oven. Referring to Figure 1, a microwave oven comprises a cooking chamber 11, a magnetron 16 located in an electrical component compartment 14 and which radiates microwaves into the cooking chamber 11, a high-voltage transformer 13 which powers the magnetron 16, a heating element 20 mounted to an upper portion of the cooking chamber which cooks food therein by radiant and convection heat, a rotating tray 12 located at the bottom of the cooking chamber 11 which receives food, a shaft 43 having its upper end connected with the bottom centre of the tray 12 and its lower end extending below the cooking chamber 11, a rotation motor 42 which delivers torque to a rotary gear so as to rotate the shaft 43 and thus the tray 12, an elevation guide member 44 positioned under the shaft 43 which elevates the shaft 43, an elevation motor 41 which actuates the elevation guide member 44 so as to elevate the shaft 43, a weight sensing unit 45 placed under the elevation guide member 44 which measures the weight of food placed on the tray 12 by sensing the pressure of the shaft 43 when the shaft 43 and the tray 12 are lowered to their lowest positions, a position sensing unit 50 (a,b) which senses the height of the food placed on the tray 12, and a control means not shown which compares the height of the food sensed by the position sensing unit 50 (a,b) with a reference height and controls the operation of both of the heating element 20 and the elevation motor 41. Referring now to Figure 2, the control circuit of the microwave oven of Figure 1 includes a controller or control means 100 which controls the operation of the microwave oven from the start of cooking to its completion, a power input unit 110 which provides the controller 100 with electrical power, a key operation unit 120 used to select the desired cooking mode and duration, a display unit 130 which displays various messages and cooking conditions during operation, a heater driving unit 140 which controls the operation of the heating element 20, an elevation motor driving unit 150 which controls the operation of the elevation motor that elevates the tray under the control of the controller 100, a rotation motor driving unit 160 which controls the operation of the rotation motor for rotating the tray, and a position sensing unit 50 for sensing as to whether or not food placed on the tray is in contact with the heating element. Referring to Figure 1 and Figure 2, the operation of the present invention is as follows. The tray 12, which is elevated or lowered by the elevation means 41, is loaded with food. The position sensing unit 50, which consists of a light emitting portion 50a and a light receiving portion 50b, then senses the height of the food. The control means 100 then compares the height of the food sensed by the position sensing unit 50 with a reference height, and then controls the operation of both of the heating element 20 and the elevation means 41 based on the result. The operation of the inventive apparatus that controls the operation of the heating element will now be described in more detail. As shown in Figure 1, the microwave oven is divided into the cooking chamber 11, defined by the metallic cabinet 10, and the electrical component compartment 14 which accommodates the oven's electrical components. The magnetron 16, located in the electrical component compartment 14, radiates microwaves into the cooking chamber 11 to cook food therein, and the high voltage transformer 13 powers the magnetron 16. The heating element 20, which supplements the microwave cooking process with radiational heat, is mounted to the upper portion of the cooking chamber 11. The tray 12 at the bottom of the cooking chamber 11 rotates and/or raises and lowers food loaded thereon. An terminal 20a, which supplies power to the heating element 20, is connected at one end to the heating element 20 and at the other end is connected, in the electrical component compartment 14, to a horizontal shaft 21 which extends outside the cabinet 10. An external handle 22 coupled to the horizonal shaft 21 can be used to swing the heating element 20 between horizontal and vertical configurations. The shaft 43 extends vertically into the cooking chamber 11 and its upper end is connected to the centre of the bottom of the tray 12. The shaft's 43 lower end extends below the cooking chamber 11. The rotation motor 42 drives two rotary gears, one of which is connected to the shaft 43 and the other of which is connected to the rotation motor, to rotate the tray 12 by rotating the shaft 43. The elevation guide member 44 is mounted under the lower end of the shaft 43 and, when the elevation motor 41, activates the elevation guide member 44, the member 44 acts on the shaft 43 to raise both it and the tray 12. The weight sensing unit 45 is installed under the elevation guide member 44 and measures the weight of the food loaded on the tray 12 by sensing the pressure applied to itself by the shaft 43 when the shaft 43 and the tray 12 in their lowest position. The weight sensing unit 45 compares an output signal frequency during its initial state with the output signal frequency when the shaft 43 has been lowered to its lowest position, and calculates the difference between them to determine the weight of the food on the tray 12. The position sensing unit 50 measures the height of the food loaded on the tray 12 and the controller 100 compares the height of the food sensed by the position sensing unit 50 with a reference height and then controls the operation of both of the heating element 20 and the elevation motor 41. The operation of the control circuit will now be described in more detail. Referring to Figure 2, the controller 100 controls the operation of the microwave oven from the start to the completion of cooking and the power input unit 110 supplies the controller 100 with electric power. The user inputs the desired cooking mode and duration to the microwave oven using the key operation unit 120. The display unit 130 displays the current state of the oven and the cooking mode so that the user may check the progress of the procedure. The heater driving unit 140 controls the operation of the heating element 20 and the elevation motor driving unit 150 controls the operation of the elevation motor 41 in dependence on a control signal sent from the controller 100. The rotation motor driving unit 160 controls the operation of the rotation motor 42, and the position sensing unit 50 detects whether or not the heating element 20 is in contact with the food on the tray 12. The position sensing unit 50 comprises photoemitter 50a and a photosensor 50b positioned opposite each other on respectively the right and left inside walls of the cooking chamber. The photosensor 50b is positioned to detect light emitted by the photoemitter 50a to detect thereby when the food in the cooking chamber 11 reaches the level of the position sensing unit 50. The photoemitter and the photosensor are positioned slightly below the level of the heating element in order to sense when food on the tray is in contact with the heating element. If food on the tray 12 is piled up to the height where it is in contact with the heating element 20, the photosensor 50b does not receive light from the photoemitter 50a, because the emitted light is blocked by the food. Thus, the controller 100 determines that the heating element 20 is in contact with the food. Referring again to Figures 1 and 2, the method of sensing the imminent contact of the heater 20 with the food using the position sensing unit 50 will be now described in detail. First, after applying electric power to the microwave oven, the user selects a desired cooking mode. At this time, if the cooking mode requires the operation of the heating element 20, the position sensing unit 50 determines the height of the food, and the heater 20 is turned on. The tray 12 starts to rotate once the cooking process begins. After a predetermined time has elapsed after the tray 12 has been to rotate, the elevating motor 41 is operated to raise the tray 12 to a predetermined height. At this time, the position sensing unit 50 determines if the heating element is in contacted with food on the tray 12. If photosensor 50b does not receive light from the photoemitter 50a due to the presence of the food, the controller 100 determines that the food is piled on the tray to the point that it is in contact with the heating element 20. The controller 100 then turns off both the heater driving unit 140 and the elevation motor driving unit 150 and then issues a messages through the display unit 130 which informs the user of the situation. The user can then restart the cooking process after changing the quantity or position of the food. In another embodiment, the sensor 50 is positioned so that the controller 100 reacts before food actually touches the heating element. In all other respects this embodiment is identical to that described above. According to the present invention, if any cooking mode is selected which requires the operation of a heating element, the heating element will be turned off if it is in contact with the food, or is about to be contacted by food, and the user will then be informed. Therefore, the present invention can reduce or eliminate troubles due to the failure of cooking or the contamination of the heating element.

Claims: A microwave oven comprising a cooking chamber (11), a elevatable tray (12) for supporting food to be cooked, a heating element (20) located towards the top of the chamber and control means (100), characterised by sensing means (50a, 50b) for sensing the presence of food in contact with or in close proximity to the heating element, the control means being responsive to the sensing means to terminate a cooking operation of the oven when food in contact with or close proximity to the heating element. An oven according to claim 1, wherein the sensing means comprises a photoemitter (50a) and a photosensor (50b). An oven according to claim 2, wherein the photoemitter (50a) and the photosensor (50b) are mounted opposite each other across the chamber. An apparatus for controlling the operation of a heater for a microwave oven which uses microwaves generated by a magnetron and heat generated by a heater to cook food, said apparatus comprising: a tray on which food to be cooked is loaded;elevating means for elevating and lowering said tray; position sensing means for determining the height of the food loaded on said tray; and control means for comparing the height of the food sensed by said position sensing means with a reference height and then controlling said heater and said elevating means based on the result obtained by said position sensing means. An apparatus for controlling the operation of a heater for a microwave oven as set forth in Claim 4, wherein said position sensing means comprises a photosensor having a light emitting portion and a light receiving portion. An apparatus for controlling the operation of a heater for a microwave oven as set forth in Claim 4, wherein said position sensing means are oppositely installed on both the inside walls of the cooking chamber. An apparatus for controlling the operation of a heater for a microwave oven as set forth in Claim 5, wherein said photosensor is positioned so that said light receiving portion does not receive light generated by said light emitting portion, when said food is elevated beyond a reference height and approaches said heater. An apparatus for controlling the operation of a heater for a microwave oven as set forth in Claim 4, wherein said control means outputs control signals which turn off said elevating means, when said food is elevated beyond the reference height. An apparatus for controlling the operation of a heater for a microwave oven as set forth in Claim 4, wherein said reference height is the height where said food contacts with said heater. An apparatus for controlling the operation of a heater for a microwave oven as set forth in Claim 4, wherein in case said position sensing means detects the contact of said food with said heater, said control means outputs a message to the user through a display.

Inventors: CHOI WON-WOO, HONG SEOK-WEON, KANG KWANG-SEOK, CHOI, WON-WOO, HONG, SEOK-WEON, KANG, KWANG-SEOK
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1997
Main classification: H05B6
IPC codes: H05B6, F24C7, F24C15, H05B11