Weight sensing device for a microwave oven

Abstract: A device for sensing the weight of food to be cooked in a microwave oven has a bracket (20) supporting a motor assembly (10) having a motor axis (11). The motor axis (11) supports and is arranged to rotate a tray (6) in which food to be cooked is placed. The motor axis (11) is thus subjected to the weight of the food. The bracket (20) has two extending pieces (24a, 24b) which are separated from and spaced above its bottom (21). An upper electrode plate (30) is fixed to the extending pieces (24a, 24b) whilst a printed circuit board (40) is fixed to the bottom (21). The printed circuit board (40) is thus spaced from the upper electrode plate (30). The printed circuit board (40) carries a lower electrode plate (41) which is thereby spaced from the upper electrode plate (30) to form a capacitor whose capacitance is used to determine the weight of the food in the tray (6). An initial food weight is set by varying the overlapping area of the two electrode plates (30, 41) by sliding the printed circuit board (40) in its own plane. The weight of the food is then sensed by alterations in capacitance caused by the depression of an elastically supported area (32) of the upper electrode plate (30) by way of the motor axis (11). Since no weight is applied to the printed circuit board (40) deformation of the printed circuit board is prevented. Furthermore, the distance between the two electrode plates may be maintained constant.

Description: The present invention relates to a weight sensing device for a microwave oven, for example, to a device for sensing food weight in a microwave oven. A microwave oven is an appliance for cooking foods using microwaves. The microwaves are radiated from a magnetron to expose the food in a cooking chamber for cooking. The intensity and exposure time of the microwaves may be adjusted in dependence upon the weight of the food to be cooked. To achieve this automatically, methods for sensing food weight using piezoelectric elements and the capacitance between electrode plates have been utilised. EP-A-0299395 shows an example of an automatic weight detecting device for a microwave oven. Where a capacitance method is used, the capacitance is varied according to the food weight. The frequency or intensity of a high frequency is adjusted according to the variation of the capacitance, and a microcomputer adjusts the intensity and the exposure time of the microwaves to cook the weighed food appropriately by way of microwave exposure. Therefore, the sensing of the food weight is the most important thing in order to cook the food appropriately. U.S. Patent No. 5,349,138 discloses a food weight sensing device in which food to be cooked is placed on a tray of the microwave oven thereby lowering a supporting bracket together with a movable electrode plate. Thus, the distance between the movable electrode plate and a fixed electrode plate is reduced to vary the capacitance therebetween. A microcomputer reads the food weight according to the capacitance variation to thereby sense the food weight. EP-A-0302396 describes a weight sensing device for a microwave oven comprising two electrode plates individually supported to be spaced one above the other by a fixed distance when no weight is applied, the upper one of said two electrode plates having an elastically supported area arranged to be depressed in dependence upon an applied weight to change the distance between two electrode plates and thereby provide a measure of the applied weight. However, repeated use of known devices for sensing the food weight may induce variations in the initially set distance between the two electrode plates and a problem of difficult detection of the accurate food weight will then occur. Moreover, where the fixed electrode plate is carried by a printed circuit board (PCB), the PCB might be destroyed after repeated deformation. According to the present invention a weight sensing device as defined above is characterised in that one of the two electrode plates is slidable in its own plane to enable relative horizontal movement of the two electrode plates whereby an initial weight reading can be set by varying the overlapping area of the two electrode plates. For example, the initial weight may be reset by sliding the lower electrode plate to adjust its overlapping area with the upper electrode plate after repeated use of the microwave oven so that the device senses the food weight according to the varied vertical distance between the upper and lower electrode plates. An embodiment of a device of the invention is able to set the initial weight by varying the overlapping area of the two electrode plates. This enables the initial distance between the two electrode plates to be kept constant and therefore enables the food weight to be accurately sensed even after repeated use. In an embodiment, the lower one of said electrode plates is printed on a printed circuit board, and the printed circuit board is mounted to be slidable in its own plane whereby the overlapping area of the two electrode plates is variable. Said elastically supported area of said upper electrode plate may be a substantially rigid area fixed to said plate by elastic means. In an embodiment, said elastically supported area of said upper electrode plate is an area within said upper electrode plate which is separated from said upper electrode plate along three sides. In this latter case, the electrode plate and the elastically supported area thereof are preferably of elastic material, for example, manufactured as a plate spring. Preferably, a bracket for supporting said two electrode plates is provided, said bracket having first surface means for supporting said upper electrode plate and second surface means for supporting said lower electrode plate, wherein said first surface means are spaced above said second surface means. In an embodiment, said bracket is provided with two side walls and a bottom, the front side and the rear side of the bracket being open, and the bottom forming said second surface means supporting the lower electrode plate. Preferably, said weight sensing device further comprises a motor assembly mounted on said bracket, the motor assembly having a motor axis for supporting a food tray, and wherein the upper electrode plate is fixed to said bracket below said motor assembly to be in contact with said motor axis. In an embodiment of the invention, the bracket is provided with first extending pieces extended from upper and rear portions of both side walls of the bracket, and the motor assembly is mounted on the first extending pieces. The bracket is provided with second extending pieces extended from front and lower portions of the side walls, the second extending pieces are vertically higher than the bottom of the bracket, and the upper electrode plate is fixed to the second extending pieces. The bracket is provided with guides bent at a rear portion of the bottom, for guiding the printed circuit board. The bracket is further provided with shoulders extended from upper portions of the side walls of the bracket and a housing of a microwave body is combined with the shoulders. The upper electrode plate is provided with a body and the elastic supporting piece formed in a region from a centre portion to a rear portion of the body. For example, the elastic supporting piece is separated from both sides and a front of said body and is elastically supported by the rear portion of said body, and a concavity is formed at the front portion of said elastic supporting piece to introduce the end portion of said motor axis. The printed circuit board is provided with longitudinal holes at both sides from a centre thereof so that the printed circuit board may slide between the upper electrode plate and the bottom of the bracket forwardly and backwardly. The overlapping area is adjusted by sliding the printed circuit board at a state that screws are inserted in the longitudinal holes. In a device for sensing food weight in the microwave oven of the present invention, if the PCB slides back and forth centring around a bolt inserted in the longitudinal hole of the PCB, the overlapping area between the upper and lower electrode plates changes. Accordingly, the capacitance changes. At this time, the overlapping area is adjusted to the initial weight of zero and the PCB is fixed to the bracket by tightening up the bolt passing through the longitudinal hole. Thus, the initial weight is set. After setting the initial weight, when food is placed on a tray of the microwave, the elastic supporting piece is lowered by receiving the food weight via the motor axis of the motor assembly which supports the tray. Accordingly, the distance between the upper and lower electrode plates is narrowed to increase the capacitance of a capacitor comprised of the fixed electrode plate and the movable electrode plate. Using a frequency variation due to the increase of the capacitance, a microcomputer senses the food weight. By using an embodiment of a device for sensing food weight of the invention in a microwave oven, the initial weight of the food can be set using the overlapping area of the upper electrode plate with the lower electrode plate. Further, since the initial weight is not changed, the weight of the food can be accurately measured to cook appropriately. In addition, the PCB is not deformed nor destroyed after repeated usage of the device for sensing the food weight and the food weight is accurately delivered to the upper electrode plate. Preferably, said bracket is formed in one body. In an embodiment, said upper electrode plate is formed so as to have a plate spring shape. The end portion of said motor axis may be inserted into a concavity of said elastic supporting piece in said upper electrode plate. Embodiments of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a cross-sectional view of a substantially conventional device for sensing food weight in a microwave oven; Figure 2 shows an exploded perspective view of a device for a microwave oven for sensing food weight according to an embodiment of the present invention; Figure 3 is a perspective view of the device of Figure 2 showing a PCB inserted between a movable electrode plate and the bottom of a bracket for setting an initial weight; and Figure 4 is a cross-sectional view showing the device of Figure 2 in an assembled state with a housing of a microwave oven. In the drawings, the same reference numerals are used for the same or similar components. A conventional capacitance-type weight sensing device is provided with a fixed electrode plate which is printed in a printed circuit board (PCB) including a control circuit having a resistance-capacitor (RC) oscillating circuit. A movable electrode plate, for example, fabricated by use of a plate spring, has a position arranged to change in dependence upon the food weight. The distance variation between the two electrode plates induces a variation in the capacitance between the two electrode plates. A frequency of the high frequency is varied by the capacitance variation between the fixed electrode plate and the movable electrode plate. The microcomputer senses the frequency of the changed high frequency and reads out the food weight corresponding to the sensed frequency from a previously prepared look-up table. Then the microcomputer controls the microwave intensity and the cooking time in dependence upon the food weight. The food weight in such a microwave oven is detected utilizing the following equation (1) : C∝εA/d wherein, C represents the capacitance, ε represents a dielectric constant of air, A represents the overlapping area of the electrode plates and d represents the distance between the electrode plates. As shown in equation (1), when the overlapping area A is constant, the capacitance C is inversely proportional to the distance d between the fixed electrode plate and the movable electrode plate. Similarly, when the distance d is constant, the capacitance C is proportional to the overlapping area A between the fixed electrode plate and the movable electrode plate. That is, when the distance d becomes shorter and the overlapping area A becomes wider, the capacitance C increases. Figure 1 shows a cross-sectional view of a substantially conventional device for sensing food weight in a microwave oven, the device utilizing the distance variation between a fixed electrode plate and a movable electrode plate. As is illustrated in Figure 1, the device for sensing the food weight in the microwave oven includes a bracket 2 assembled with a motor 1, having a motor axis 1a. The bracket 2 is provided to transfer the weight of food which is placed on a tray 6. A movable electrode plate 3 provided with an elastic supporting piece 3a is positioned under the motor 1 and is arranged to receive the food weight from the motor axis 1a. A fixed electrode plate 5 is spaced beneath the movable electrode plate 3. To maintain a constant distance d between the fixed and movable electrode plates, the centre portion of the fixed electrode plate 5 is supported by a screw 8 so that it is a predetermined distance from the bottom of the bracket 2, and spacers 4 maintain the distance d between the movable electrode plate 3 and the fixed electrode plate 5. The device shown in Figure 1 for sensing food weight operates as follows. Firstly, the distance d between the fixed electrode plate 5 and the movable electrode plate 3 is adjusted by use of the screw 8 to set the initial weight. The initial weight corresponds to the weight of an empty tray without food, i.e., to that when the food weight is zero. After setting of the initial weight, the elastic supporting piece 3a is bent due to food weight on the tray 6 and this changes the distance d between the movable electrode plate 3 and the fixed electrode plate 5. Thus, the capacitance C between the two electrode plates 3 and 5 changes (see equation (1) above). The frequency of the high frequency is thus changed by the change in the capacitance C. A microcomputer (not shown) senses the change in frequency and is arranged to read the corresponding food weight from a look-up table provided. The microcomputer causes the radiation of microwaves with an appropriate intensity for an appropriate time suitable for cooking the food whose weight has been determined. However, the food weight which is the factor for bending the elastic supporting piece 3a acts as a force to pressurize the PCB and a repulsive force opposed to this force is generated at the centre portion of the PCB which is supported by the screw 8. Due to the forces applied in this manner, the PCB is deformed into a shape having a convex centre portion. Repeated generation of the PCB deformation results in a variation of the initially set distance d between the electrode plates 3 and 5. That is, the initial weight when no food is placed on the tray 6 deviates from zero. Figure 2 shows an exploded perspective view of a device according to an embodiment for sensing food weight. The device shown in Figure 2 is for use in a microwave oven and includes a bracket 20 having two side walls 22a and 22b and a bottom 21. The front and rear sides of bracket 20 are open. A PCB 40 is mounted on the bottom 21 of the bracket 20, and a movable electrode plate 30 is fixed to second extending pieces 24a and 24b which are extended from the side walls 22a and 22b of the bracket 20. The bracket 20 is integrally formed by bending the side walls 22a and 22b of the bracket 20 from the bottom 21. Shoulders 26a and 26b are bent and extended from the upper portion of the side walls 22a and 22b of the bracket 20. A plurality of first through holes 26c is formed in the shoulders 26a and 26b. The bracket 20 is combined with a housing 7 of a microwave oven body (see Figure 4) by means of bolts extending through the first holes 26c. In addition, the bracket 20 is provided with first extending pieces 23a and 23b on which a motor assembly 10 is mounted. First extending pieces 23a and 23b are bent and extended from the rear and upper portion of the side walls 22a and 22b of the bracket 20. The motor assembly 10 is mounted on the first extending pieces 23a and 23b by means of bolts which extend through a sixth through hole 12 and second through holes 23c and 23d. The second extending pieces 24a and 24b of the bracket 20 are bent and extended from the front and lower portion of the side walls 22a and 22b of the bracket 20. The second extending pieces 24a and 24b are separated from the bottom 21 of the bracket 20 and are vertically higher than the bottom 21. Third through holes 24c and 24d are formed in the second extending pieces 24a and 24b. The movable electrode plate 30 is provided with a body 31 and an elastic supporting piece 32 which is formed in a region between centre and rear portions of the body 31. A space 34 is formed between the front end of the elastic supporting piece 32 and the front end of the body 31. The elastic supporting piece 32 is separated from both sides of the body 31 and from the front side of body 31. The elastic supporting piece 32 is elastically supported by the rear portion of the body 31. A concavity 33 is formed at the front centre portion of the elastic supporting piece 32. An end portion of a motor axis 11 of the motor assembly 10 is inserted into the concavity 33 of the elastic supporting piece 32 and the food weight is transferred to the elastic supporting piece 32 by way of the motor axis 11 without sliding. Fifth through holes 35a and 35b are formed in both sides of the body 31 of the movable electrode plate 30. The movable electrode plate 30 is fixed to the second extending pieces 24a and 24b of the bracket 20 by bolts extending through the fifth through holes 35a and 35b and through the third through holes 24c and 24d of the second extending pieces 24a and 24b by means of bolts. Preferably, the movable electrode plate 30 is manufactured as a plate spring. The bracket 20 is provided with a pair of guides 25a and 25b which guide the PCB 40. Each guide 25a, 25b of the pair is bent and positioned at the rear portion of the bottom 21. Fourth through holes 21a and 21b by which the PCB 40 is fixed are formed in the front portion of the bottom 21. The PCB 40 is provided with a fixed electrode plate 41, which is printed at the rear portion thereof, and with longitudinal holes or slots 42a and 42b which are located at both sides from the centre portion thereof. The PCB 40 is fixed to the fourth through holes 21a and 21b in the bottom 21 of the bracket 20. The fixed electrode plate 41 and the movable electrode plate 30 together form a capacitor. The device for sensing food weight in a microwave illustrated in Figure 2 is assembled and the initial weight is set as described below. Figure 3 is a schematic view for explaining the assembling process and the setting of the initial weight. As is indicated, the motor assembly 10 is first mounted on the first extending pieces 23a and 23b of bracket 20. Then, the movable electrode plate 30 is fixed to the second extending pieces 21a and 21b of the bracket 20 by means of bolts. At this stage, the PCB 40 is guided by the pair of guides 25a and 25b of the bracket 20 and is slidably inserted into the space formed between the movable electrode plate 30 and the bottom 21 of the bracket 20. When the PCB 40 has been slidably inserted into the space between the movable electrode plate 30 and the bottom 21 of bracket 20 it is secured by bolts inserted into the longitudinal holes 42a and 42b therein and into the fourth through holes 21a and 21b in the bracket 20. It will be appreciated that the length of the longitudinal holes 42a and 42b can enable the PCB 40 to be slid back and forth relative to the bolts inserted therein. This changes the overlapping area A between the PCB 40 and the movable electrode plate 30. When the overlapping area A is changed in this way, the capacitance C of the capacitor is also changed as shown by equation (1) given above. Therefore, after adjusting the overlapping area A so that the initial weight is zero, the bolts inserted into the longitudinal holes 42a and 42b are tightened to fix the PCB 40 to the bracket 20. Now the initial weight setting is completed. After setting the initial weight, the device is coupled to the housing 7 of a microwave oven body using the shoulders 26a and 26b of the bracket 20 as is shown in Figure 4. Figure 4 is a cross-sectional view showing the combined state of the housing 7 of a microwave body with the device of Figures 2 and 3. As is shown in Figure 4, when food (not shown) is placed on the tray 6, the elastic supporting piece 32 is depressed by the motor axis 11 of the motor assembly 10 which supports the tray 6. As the elastic supporting piece 32 is lowered, the distance between the fixed electrode plate 41 and the movable electrode plate 30 is narrowed and the capacitance C between the two electrode plates 30 and 41 is increased as can be seen from equation (1). The increased capacitance C changes the frequency of the high frequency and the microcomputer (not shown) senses this change to read the food weight from the previously prepared look-up table. The food cooking is controlled by the microcomputer. Further, when the distance between the fixed electrode plate 41 and the movable electrode plate 30 is changed to alter the capacitance of the capacitor consisting of the fixed electrode plate 41 and the movable electrode plate 30 after repeated use of the microwave oven, the initial weight deviates from zero. At this time, the initial weight needs to be reset by changing the overlapping area of the fixed electrode plate 41 and the movable electrode plate 30. In such a case, the initial weight of the device for sensing the food weight in a microwave oven can be easily reset by slightly sliding backwards or forwards the PCB 40 which is slidably fixed to the bracket 20. Thus, resetting of the initial weight may be easily performed. The initial weight of the food can be set by adjustment of the overlapping area of the fixed electrode plate relative to the movable electrode plate. Since the fixed electrode plate and the movable electrode plate are spaced and fixed, the initial distance between the two electrodes is constant, and the initial weight is not changed. In addition, in a device of the invention, deformation or destruction of the PCB after repeated use can be prevented. Further, since the food weight is accurately delivered to the movable electrode plate, the food weight can be accurately sensed. Accordingly, a device of the invention enables appropriate cooking to be accomplished through accurate sensing of the food weight. The present invention has been described above with reference to the embodiment illustrated. However, it will be appreciated that the invention is not limited to that embodiment, and that various changes and modifications can be made without departing from the scope of the invention as defined in the appended claims.

Claims: A weight sensing device for a microwave oven comprising two electrode plates (30,41) individually supported to be spaced one above the other by a fixed distance when no weight is applied, the upper one (30) of said two electrode plates having an elastically supported area (32) arranged to be depressed in dependence upon an applied weight to change the distance between the two electrode plates (30, 41) and thereby provide a measure of the applied weight, characterised in that one (41) of the two electrode plates (30, 41) is slidable in its own plane to enable relative horizontal movement of the two electrode plates (30, 41) whereby an initial weight reading can be set by varying the overlapping area of the two electrode plates. A weight sensing device as claimed in Claim 1, wherein the lower one (41) of said electrode plates is printed on a printed circuit board (40), and wherein the printed circuit board (40) is mounted to be slidable in its own plane whereby the overlapping area of the two electrode plates is variable. A weight sensing device as claimed in Claim 1 or Claim 2, wherein said elastically supported area (32) of said upper electrode plate (30) is an area within said upper electrode plate which is separated from said upper electrode plate along three sides. A weight sensing device as claimed in any preceding claim, further comprising a bracket (20) for supporting said two electrode plates (30, 41), said bracket (20) having first surface means (24a, 24b) for supporting said upper electrode plate (30) and second surface means (21) for supporting said lower electrode plate (41), wherein said first surface means (24a, 24b) are spaced above said second surface means (21). Weight sensing device as claimed in Claim 4, wherein said bracket (20) is provided with two side walls (22a, 22b) and a bottom (21), the front side and the rear side of the bracket being open, and the bottom (21) forming said second surface means supporting the lower electrode plate (41). A weight sensing device as claimed in Claim 4 or Claim 5, further comprising a motor assembly (10) mounted on said bracket (20), the motor assembly (10) having a motor axis (11) for supporting a food tray, and wherein the upper electrode plate (30) is fixed to said bracket (20) below said motor assembly to be in contact with said motor axis (11). A weight sensing device as claimed in Claim 6, wherein said bracket (20) is provided with first extending pieces (23a, 23b) extended from upper and rear portions of both side walls (22a, 22b) of said bracket, and said motor assembly (10) is mounted on said first extending pieces (23a, 23b). A weight sensing device as claimed in any of Claims 4 to 7, wherein said bracket (20) is provided with second extending pieces (24a, 24b) extended from front and lower portions of its side walls (22a, 22b), said second extending pieces (24a, 24b) being vertically higher than the bottom (21) of said bracket, and said second extending pieces forming said first surface means supporting said upper electrode plate (30). A weight sensing device as claimed in any of Claims 4 to 8, wherein said bracket (20) is provided with guides (25a, 25b) bent at a rear portion of its bottom (21) for guiding the lower electrode plate (41). A weight sensing device as claimed in any of Claims 4 to 9, wherein said bracket (20) is provided with shoulders (26a, 26b) extended from upper portions of its side walls (22a, 22b) and a housing (7) of a microwave body is combined with said shoulders.

Inventors: LEE JONG CHEOL, LEE, JONG CHEOL
Applicants: DAEWOO ELECTRONICS CO LTD, DAEWOO ELECTRONICS CO., LTD
Application year: 1995
Main classification: G01G19
IPC codes: H05B6, G01G19, F24C7, G01G3, G01G7