Microwave oven with impedance matching control function

Abstract: A microwave oven includes a heating chamber, a turntable (8) provided in the oven, a magnetron (3), a waveguide (2) for guiding the microwave generated by the magnetron (3) into the heating chamber (1), an electric field sensor (10) provided at the inner wall face of the waveguide (2) or the heating chamber (1), a metal reflector (9) provided rotatably inside the waveguide (2) for adjusting the impedance of a load at the heating chamber side by its rotation angle, and a circuit responsive to an output of the electric field sensor (10) for controlling the rotation angle of the metal reflector (9). The control circuit includes a first program for controlling the rotation angle of the metal reflector (9) according to a first method within a predetermined time period from the start of operation of the microwave oven, and a second program for controlling the rotation angle of the metal reflector (9) according to a second method different from the first method after elapse of the predetermined time period. For example, in the first method, the rotation angle of the metal reflector (9) is sequentially controlled so that the output of the electric field sensor (10) is minimum, and in the second method, the rotation angle of the metal reflector (9) is fixed to an average value of the rotation angle of the metal reflector (9) of the first method. The initial rotation angle of the metal reflector (9) is determined so that impedance matching can be obtained when load of approximately 300-500cc calculated in water load is placed at the center of the heating chamber.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a microwave oven, and more particularly to the control of matching impedance of a microwave generating device and a load in a microwave oven. The invention is delimited against Japanese Utility Model Publication No. 1-255 13/Japanese Patent Laying-Open No. 55-88289. Description of the Background Art It is known that the impedance in a heating chamber varies depending upon the magnitude of load and position of the food item in a microwave oven. Because of this variance in impedance, the impedance between the magnetron and the heating chamber cannot be matched. As a result, the electric field by the microwave reflected from the heating chamber therein increases. In such a case, the power of the microwave cannot be effectively used. A microwave oven is disclosed that can adjust the position of a metal reflector for matching impedance in order to reduce the reflected electric field of the microwave (refer to Japanese Utility Model Laying-Open No. 61-100897, Japanese Patent Laying-Open No. 56-160792). Another microwave oven is disclosed that can control impedance by moving a stub and/or a metal reflector depending upon the weight of food or the specified heating time according to a cooking program (refer to Japanese Utility Model Publication No. 1-25513, Japanese Patent Laying-Open No. 55-88289). These conventional microwave ovens are directed to carry out effective operation of microwave power by closely controlling and adjusting the position of the device for matching impedance such as a metal reflector. A metal reflector is also used in a microwave oven disclosed in U.S. Patent Nr. 4,210,794. However, the metal reflector in this microwave oven is not used for impedance matching but to protect a turntable drive gear from microwave radiation, which drive gear is made of a resinous microwave absorbing material. In order to effectively carry out impedance matching by rotating a metal reflector within a waveguide, it is necessary to move the position of a metal stub provided in a rotation plate (disk) of the metal reflector in the degree of λg/4 (λg: wavelength within waveguide of microwave) where the maximum and minimum value of the voltage of the standing wave of the microwave for the microwave oven (2450MHz) appears. Therefore, the rotation plate of the metal reflector must have a diameter of approximately 40mm corresponding to λg/4. The device for matching impedance is provided in the waveguide having a high microwave electric field. In order to avoid arcing due to voltage induced by the microwave from the device for matching impedance of a metal reflector and the like, the device for matching impedance must be remote from the wall face of the waveguide by a sufficient spatial distance. Taking also into account the electric field concentration towards the end portion of the device for matching impedance, and change in the reflected wave depending upon the condition of load (light load) in the oven, the device for matching impedance should be as small as possible in order to avoid atmospheric discharge within the waveguide. Effective usage of microwave power can be expected by controlling and adjusting the position of the impedance-matching reflector for reducing the reflected power from the oven including the substance to be heated. A possible method is to continuously control and drive the impedance-matching device by a motor or the like in cooking the substance-to-be-heated. However, because the load position is always changing in a microwave oven employing a turntable mechanism, the load is great in the driving mechanism for continuous follow. In view of the specified life time of a microwave oven, there is problem in the cost and reliability of the impedance-matching device. In view of the foregoing, the usage of an impedance-matching device in a microwave oven manufactured in mass production has the following problems. In the manufacturing stage, the structure of the impedance-matching device and the structure of members thereof must be made to be applicable for mass production while optimizing the adjustable region of impedance and carrying out measures with respect to arcing within the waveguide. Although information of the food load in the oven is not obtained right after the initiation of a cooking operation, it is desirable to determine the operation mode of the impedance-matching device at the start of the operation to obtain a preferable result. Although the heated time of cooking is normally short in cooking light food of load, the time for detecting information to determine the heating mode in operation of the food in the oven by an electric field sensor or a weight sensor takes several seconds to ten several seconds which is a relatively long time. Because the heating mode in operation of the start greatly affects the heating efficiency of food of light load, the load must be determined taking into consideration the magnitude thereof. A change in impedance is expected due to difference in temperature before and after cooking the food item, or due to change in weight caused by vaporization of the food. It is therefore necessary to sense and correct it appropriately. The impedance-matching device employed in an economic type microwave oven for domestic use must have low component cost. SUMMARY OF THE INVENTION In view of the foregoing, an object of the present invention is to provide an microwave oven having an impedance-matching system of high reliability that can allow effective usage of the microwave. Another object of the present invention is to provide a microwave oven having a small impedance-matching system of high reliability allowing effective usage of the microwave. A further object of the present invention is to provide a microwave oven of a small impedance-matching system of high reliability allowing effective usage of the microwave without the possibility of arcing in a waveguide. Still another object of the present invention is to provide a microwave oven suitable for mass production having a small impedance-matching system of high reliability allowing effective usage of the microwave without the possibility of arcing in a waveguide. A still further object of the present invention is to provide a microwave oven suitable for mass production having a small impedance-matching system of high reliability allowing effective usage of the microwave and without arcing in a waveguide, and that can carry out an appropriate heating operation according to the weight of the food item. Yet a further object of the present invention is to provide a microwave oven suitable for mass production having a small impedance-matching system of high reliability allowing effective usage of the microwave without the possibility of arcing within a waveguide, and that can carry out appropriate heating operation according to the weight or cooking time of food. Yet another object of the present invention is to provide a microwave oven suitable for mass production having an economic and small impedance-matching system of high reliability allowing effective usage of the microwave without the possibility of arcing in a waveguide, and that can carry out appropriate heating operation according to the weight or cooking time of food. According to the present invention, a microwave oven includes a heating chamber having an opening through which a microwave is introduced, a turntable provided in the heating chamber for supporting a substance to be heated, a device for generating a microwave, a waveguide for guiding the microwave in a predetermined traveling direction to direct the same into the heating chamber through the opening, an electric field sensor provided in the inner wall face of the waveguide or the heating chamber, a device provided rotatably about a predetermined shaft in the proximity of the opening in the waveguide for adjusting impedance of the load at the heating chamber side by a rotation angle thereof, and a device responsive to an output of the electric field sensor for controlling the rotation angle of the impedance-adjusting device. The device for adjusting the impedance includes a rotation shaft provided in the waveguide to cross the direction of travel of the microwave by a predetermined angle, a rotation plate having a metal face attached to the rotation shaft, and a metal stub formed projecting from the metal face at the peripheral portion of the rotation plate at a predetermined angle with respect to the metal face. The above-described control device may include a memory for prestoring a table of target rotation angles of the impedance-adjusting device, a weight sensor for detecting the weight of the substance to be heated that is conveyed into the heating chamber, and a circuit for controlling the rotation angle of the impedance-adjusting device by table lookup of the target rotation angle with the detected weight as an argument. The rotation plate may be provided with its rotation axis substantially perpendicular to the direction of travel of the microwave, or at an angle greater than 0° and less than 90°. In the case of the latter, the angle of the metal stub with respect to the metal face of the rotation plate may be selected to be equal to that of the rotation axis of the rotation plate to the direction of travel of the microwave. The metal stub has a longitudinal component perpendicular to the wall face of the waveguide. The metal face rotates according to the rotation of the dielectric rotation shaft by the control means to move by a component of the direction of travel of the diameter of the metal face. Thus, the capacitance component and the induction component of the impedance can be adjusted to match the impedance. The adjustment of the induction component and the capacitance component of impedance will be described in detail. It is assumed that the metal face is parallel to the direction of travel. Because there is almost no change in the distance between the wall face of the waveguide and the metal face depending on the rotation angle about the shaft of the metal face, there is also no change in the capacitance component. However, the position of the metal stub provided at the peripheral portion of the metal face changes greatly. The microwave electric field within the waveguide is high at the middle and low at the circumference in the cross section (E plane) of the waveguide. In the vertical cross section (H plane) of the waveguide, reflected wave exists with respect to the traveling wave. Therefore, there is a high and low electric field at an interval of half (λg/2) of the wavelength (λg) in the waveguide by the microwave frequency. By the rotation of a metal face of a disk of λg/4 in diameter, for example, the metal stub moves along the circumference of the metal face, and passes the minimum point of the microwave electric field from the maximum point thereof. The control variation range of the induction component to be adjusted can be set by appropriately specifying the height of the metal stub. In accordance with the amount of load of the food to be heated, the position of the metal reflector can be controlled referring to the conditions stored in the memory. Then, in response to the intensity of the electric field of the microwave, the position of the metal reflector can be controlled to approximate the optimum control target value. Thus, an optimum matching impedance can be maintained in accordance with the state of the food load to improve the heating efficiency during cooking operation. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 13 is a sectional view showing an embodiment of a microwave oven according to the present invention. Fig. 14 is an enlarged view of the portion surrounded by a chain dotted line in Fig. 13. Fig. 15 is a perspective view showing an example of the metal reflector of Fig. 14. Fig. 16 is a perspective view showing another example of the metal reflector of Fig. 14. Fig. 17A shows a further example of the metal reflector of Fig. 14. Fig. 17B is a sectional view taken in the direction of arrow 17B-17B of Fig. 17. Fig. 18 is a perspective view of still another example of the metal reflector of Fig. 14. Fig. 19A schematically shows the angular position of a metal reflector. Fig. 19B is a Smith chart for describing the impedance varying range according to the control of the position of the metal reflector 55. Fig. 20 shows the schematic relationship between the weight of a food item and the rotation angle of a metal reflector. Fig. 21 is a portion of a flow chart showing the control of a metal reflector from the start to the end of a cooking operational mode. Fig. 22 is a portion of a flow portion showing the control of a metal reflector from the start to the end of a cooking operational mode. Figs. 23A and 23B are graphs of the output value of an electric field sensor during one rotation of the turntable over time, wherein Fig. 23A shows the case where the output value approximates an optimum control target value, and Fig. 23B shows the case where the maximum and minimum values of the output value exceed the tolerance range. Fig. 24 is a flow chart showing another embodiment of an active load matching system according to the present invention. Fig. 25 is a vertical sectional view showing an embodiment of a microwave oven according to the present invention. Fig. 26 is an enlarged diagram of the waveguide portion of Fig. 25. Fig. 27 is a perspective view of a metal reflector provided at the bottom of a waveguide. Fig. 28 is a sectional view of a metal reflector for describing change in impedance. Fig. 29 is a vertical sectional view showing another embodiment of the waveguide portion. Fig. 30 is a plan view showing the relationship between the positions of a support roller base provided at the bottom of the heating chamber and a weight sensor. Figs. 31A and 31B are waveform charts of a voltage output from a piezoelectric weight sensor. Figs. 32A and 32B are voltage waveform charts showing the output value of a weight sensor of each roller according to difference in the mounted position of food on a turntable. Fig. 33 is a graph showing the relationship between the conversion value of an output voltage with respect to weight of food load. Fig. 34 is a diagram for describing the rotation position of a metal reflector for adjusting impedance. Fig. 35 is a Smith chart plotting impedance of the heating chamber when the metal reflector is rotated, wherein a represents water load of 300cc and b represents water load of 2000cc. Fig. 36 shows the high frequency output of water loads of 2000cc and 300cc with the rotation position of a metal reflector as a parameter. Fig. 37 is a data table showing a rotation position of a metal stub (metal reflector) with respect to load weight. DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment Fig. 13 is a sectional view of a microwave oven according to a first embodiment of the present invention. Referring to Fig. 13, a microwave oven having an antenna 3-1 includes a magnetron 3, a waveguide 52 for conducting a microwave radiated from magnetron antenna 3-1 to heating chamber 1, a metal reflector 55 for impedance matching, a motor 58 for rotating metal reflector 55, and a circuit 68 for controlling motor 58. A turntable 72 and a roller 73 for supporting turntable 72 are provided in heating chamber 1. A turntable motor 74 is provided outside heating chamber 1. A weight sensor 75 is provided at a passing point of roller 73. A weight detection signal of weight sensor 75 is provided to control circuit 68. Metal reflector 55 adjusts impedance of the heating chamber side for impedance matching. Metal reflector 55 is provided within waveguide 52 and is not seen at the time of usage of the microwave oven by an opening cover 1-2. Magnetron 3 is provided in an outer box 70 of the microwave oven. A microwave radiated from antenna 3-1 passes through waveguide 52 as a traveling wave to be radiated into heating chamber 1 through opening cover 1-2 to reach the food load to be absorbed. According to the state of the food load, a portion of the microwave returns to the interior of waveguide 52 as a reflected wave. The standing wave mode in waveguide 52 changes according to the ratio of the reflected wave to the traveling wave and the state of face thereof to affect the operation efficiency of magnetron 3. In the present embodiment, impedance at the side of the heating chamber seen from magnetron 3 alleviates the influence due to the size of the food load, so that the operation efficiency of the magnetron can be maintained at a high level. Fig. 14 is an enlarged view of the portion XIV surrounded by a dash dot line of Fig. 13. Referring to Fig. 14, metal reflector 55 is formed of a non-magnetic metal. Metal reflector 55 is fixed to a dielectric rotation shaft 56 formed of a low dielectric constant of dielectric material such as ceramics and plastic. Dielectric rotation shaft 56 is connected to a driven gear 57 outside waveguide 52. Driven gear 57 has its position of angle controlled by motor 58. A hole is formed in the proximity of the circumference of driven gear 57. A photo interrupter 59 is provided corresponding to the position of this hole. Photo interrupter 59 detects the position of the base angle of metal reflector 55. Motor 58 is a compact generalized type stepping motor dividing one rotation into 20 steps. By combining a reduction gear, a general compatible direct current motor may be used as motor 58. In this case, a photo interrupter can be added, whereby the rotation angle of metal reflector 55 can be set to 20 steps per rotation or 40 steps per rotation by an additional photo interrupter and a rotation hole disk attached to a motor shaft. Control circuit 68 includes an A/D converter 62, a logic operation circuit 63, a memory 64, a timing circuit 65 and a motor driving circuit 66. An A/D converter 62 converts the analog signal indicating the level of the electric field detected by electric field sensor 10 into a digital signal. Logic operation circuit 63 processes the digital data from A/D converter 62 according to the flow charts of Figs. 21 and 22 which will be described afterwards. Memory 64 stores a tolerable range of the optimum control target value of an electric level selected in advance so that the reflected wave takes a minimum value. Timing circuit 65 generates a timing signal for controlling the A/D conversion timing of A/D converter 62 and the operation timing of logic operation control circuit 63. Motor driving circuit 66 responds to the operation result of logic operation control circuit 63 to control the rotation angle of motor 58. Electric field sensor 10 fixed to the outer side 52-2 of the terminal end of waveguide 52 detects the intensity of the electric field in waveguide 52. Electric field sensor 10 senses the level of the microwave leaking from a hole of approximately 4.5mm diameter provided substantially at the center of the termination end portion. The termination portion of waveguide 52 is located at 1/8 the distance of the wavelength in the waveguide from antenna 3-1. Electric field sensor 10 includes an electric field detecting antenna, a high frequency diode, a high frequency capacitor and a lowpass filter, as in a conventional high frequency detection circuit. A microwave is detected by the electric field detection antenna to be detected by the high frequency diode to be smoothed by a high frequency capacitor, whereby microwave noise is removed by the lowpass filter. Thus, a DC ripple voltage indicating the intensity of the electric field is provided from electric field sensor 10. The A/D conversion of A/D converter 62 is carried out at a timing delayed by approximately 4m seconds to 4.5m seconds from the voltage zero cross point of the power source voltage where the maximum power is obtained in the oscillating operation of magnetron 3. The obtained data of the electric field level is supplied to logic operation control circuit 63. The stub which is the induction component is based on the angle position of metal reflector 55 which most closely approximates antenna 3-1. When light load of water of approximately 300cc is placed at the center of turntable 72, impedance at the heating chamber side is within the output maximum region of magnetron 3. A projection 52-1 is provided at the wall face of waveguide 52 to form a fixed stub for impedance adjustment. The fixed stub may have a metal projection fixed at the wall face of the waveguide. Fig. 15 is a perspective view showing an example of metal reflector 55 of Fig. 14. Referring to Fig. 15, metal reflector 55 includes a metal disk 81, and a metal plate 82 provided at the circumferential edge of metal disk 81. Metal disk 81 is formed of a non-magnetic metal such as aluminum whose surface loss of microwave is low. Metal disk 81 has its center portion supported and fixed to a dielectric rotation shaft 56 formed of a low dielectric constant material such as ceramics and plastic. The diameter of metal disk 81 is 30mm-40mm which approximates the wavelength of the microwave of approximately λg/4. Although the height of metal plate 82 depends on the height of waveguide 52, a height of 15mm-20mm approximating λg/8 of the wavelength of the microwave is selected. The formation of metal reflector 55 as shown in Fig. 15 is set forth in the following. A metal plate 82 is formed at the end of metal disk 81, and metal plate 82 is bent. Metal disk 81 mainly becomes the capacitance component of impedance, and metal plate 82 becomes mainly the induction component of impedance. Fig. 16 is a perspective view showing another example of metal reflector 55. Metal reflector 55a shown in Fig. 16 includes a metal disc 83, and a metal cylindrical stub 84 attached to metal disc 83. Metal disc 83 and metal cylindrical stub 84 mainly become the capacitance component and the induction component, respectively, of the impedance. Figs. 17A and 17B show a further example of metal reflector 55. Metal reflector 55b of Figs. 17A and 17B include a metal disk 88, a metal face 86, an edge 87 and a support portion 85. Metal disk 88 and metal face 86 mainly become the capacitance component and the induction component of the impedance component. The end of supporting portion 85 is attached to dielectric rotation shaft 56. This facilitates the fixing of metal reflector 55b to dielectric rotation shaft 56, and impedance can be adjusted a further great range. Fig. 18 is a perspective view of a metal reflector of a still another example. Metal reflector 55c of Fig. 18 is similar to that of Fig. 15 except that a metal disk 89 is added to metal reflector 55. Thus, the electrostatic coupling condition of metal reflector 55c and the sidewall of the waveguide can be changed from that of Fig. 15. Referring to Fig. 19A, the base position of metal reflector 55 (Fig. 15), i.e. the angle position of metal reflector 55 where metal plate 82 is most close to the magnetron is "P0". 180° is divided into 10 resulting in 18° for each position, and the angle position of metal reflector 55 corresponding to each angle is set as "P1" to "P10". The impedance at the heating chamber side in rotating metal reflector 55 from the base angle position to 180° is plotted on the Smith chart in Fig. 19B. Referring to Fig. 19B, curve (a) shows the case where a water load of 300cc is placed at the center of the turntable, curve (b) shows the case where a water load of 2000c is placed at the center of the turntable, and curve (c) shows the case of no load. The hatched area on the Smith chart indicates the maximum output region of the magnetron. The standard state is when metal reflector 55 is at its base angle position "P0", and when a light load corresponding to 300cc of water load is placed at the center of turntable 22. The position and height of fixed stub 52-1 (Fig. 14) is determined under this standard state such that impedance in heating chamber 1 is within the region when maximum output is obtained such as point "P0" of curve (a) on the Smith chart. If the load in heating chamber 1 is increased, the range of impedance change in the heating chamber side due to rotation of metal reflector 55 changes. As a result, the optimum angle position of metal reflector 55 becomes the state of "P10" which is approximately 180° rotated from the base position, in the case of load of 2000cc calculated in water load, as shown in curve (b) of Fig. 19B. The output level of electric field sensor 10 is obtained by experiment in advance under the state where electric field sensor 10 is provided at the end portion of the waveguide as shown in Fig. 14 and magnetron 3 operated under the conditions where the load is set so that the reflected wave is minimum. This output level is the reference value Eref of the output level. Reference value Eref is described in detail hereinafter. The output level of electric field sensor 10 correlates to the intensity of the electric field in the proximity of the inner wall face of waveguide 52. The condition for no reflected wave is when the impedance of the heating chamber side seen from magnetron 3 coincides with the impedance of magnetron 3 which is the microwave generating source. Since the output level of electric field sensor 10 approximates the reference value Eref, this reference value Eref is the optimum control target value. The impedance at the heating chamber side changes according to change of the food load within heating chamber 1, and takes a value greater or smaller than the impedance of magnetron 3. Accordingly, the electric field level of the standing wave in the proximity of electric field sensor 10 also becomes higher or lower. The output level of electric field sensor 10 also takes a value higher or lower than the reference value Eref. The middle region of the Smith chart of Fig. 19B is where there is no reflected wave. The standing wave ratio is 1 at that middle region. Therefore, by controlling the angle position of metal reflector 55 so that the output level of electric sensor 10 approximates the reference value Eref, magnetron 3 can be operated in an optimum impedance region. Referring to Fig. 20, it is presumed that a light load is to be heated right after initiation of a cooking heating operation regardless of the weight of the food in the control of heating cooking. Position "P0" is selected for the angle position of metal reflector 55. After weight information of food is received from weight sensor 75, control circuit 68 rotates metal reflector 55 promptly to an optimum angle position according to the relation shown in Fig. 20. Referring to Figs. 21 and 22, control of the angle position of metal reflector 55 will be described. In Figs. 21 and 22, Eave is the average value of the detected level of electric field sensor 10 during one rotation of turntable 22, Edif is the absolute value of the difference between the average value Eave and the reference value Eref, Ealw is the tolerance range with respect to the absolute value Edif, and Epre is the preceding measured Edif value. At step S201, the angle position of metal reflector 55 is moved to the base position "P0". At step S202, a pressure-sensitive signal (weight data) is entered from weight sensor 75. At step 203, a provisional control angle of metal reflector 55 is determined according to the relationship of Fig. 20. At step S204, metal reflector 55 is moved to that angle position. Referring to Fig. 22, the reflected electric field level detected by electric field sensor 10 is converted into digital data by A/D converter 62, and integrated by logic operation control circuit 13 at step S206. At step S207, determination is made whether the cooking time has ended or not. If the cooking time has ended, the control proceeds to step S208 to cease and end the cooking operation. If the cooking time has not yet ended, the control proceeds to step S209 where determination is made whether turntable 72 has made one rotation. If determination is made that one rotation has not been made, the control returns to step S206. Otherwise, the control proceeds to step S210. At step S210, the average value Eave is obtained from the integrated result of the output data of A/D converter 62 during one rotation of turntable 72. At step S211, the absolute value Edif which is the difference between the average value Eave and the reference value Eref is obtained. At step S212, determination is made whether the absolute value Edif is smaller than tolerable range Ealw. If Edif < Ealw, the control returns to step S206, otherwise to step S213. At step S213, determination is made whether the absolute value Edif is smaller than Epre. IF Edif > Epre, the control proceeds to step S214. At step S214, an instruction is sent to motor driving circuit 66 to reverse the direction of rotation of metal reflector 55. If not Edif > Epre, the control proceeds to step S215 where an instruction is sent to motor driving circuit 66 to rotate metal reflector 55 in the current specified direction. The initial direction of rotation of metal reflector 55 is assumed to be clockwise. A sufficient large value is set as the initial value for Epre. At step S216, the Epre held in memory 64 is replaced by a newly calculated absolute value Edif. At step S217, motor 58 is driven to rotate metal reflector 55 for one step in a direction determined at step S214 or S215. Then, the control returns to step S206. It is to be noted that the output value of electric field sensor 10 changes during one rotation of turntable 72, as shown in Figs. 23A and 24B. Fig. 23A and Fig. 23B show the cases where the changing range of the output value are small and great, respectively. In the case of Fig. 23A, the output value of electric field sensor 10 approximates the reference value Eref which is the optimum control target value, so that heating of high power efficiency can be carried out. In the case of Fig. 23B, it can be appreciated from the solid line that although the average value Eave approximates the reference value Eref, each value over time is far from the reference value Eref. This means that although the average value Eave is equal, the efficiency is poor in comparison with the case of Fig. 23A. The heating efficiency can be improved by controlling the active impedance by approximating the output values Emax and Emin of electric field sensor 10 to the reference value Eref as shown in the broken line of Fig. 23B. Fig. 24 is a modification of the flow chart of Fig. 22. This flow chart is directed to improve the heating efficiency described above with reference to Figs. 23A and 23B. Steps S210A and S211A in the flow chart of Fig. 24 differs from step S210 and S211 in the flow chart of Fig. 22. Referring to Fig. 24, a microwave oven under this modification operates as follows. An output signal of electric field sensor 10 is A/D converted by A/D converter 62, and integrated for one rotation of turntable 72 by the procedures of steps S206-S209. At step S210A, the maximum value Emax and the minimum value Emin of the A/D converted data during one rotation of turntable 72 are detected. At step S211A, absolute values Edif 1 and Edif 2 of the difference between the maximum and minimum values Emax and Emin and the reference value Eref are obtained. The appropriate control state is where absolute values Edif 1 and 2 are within the tolerable range of Ealw. At step S212, determination is made whether absolute values Edif 1 and 2 are below the tolerable range, i.e. Edif 1 < Ealw, and Edif 2 < Ealw. If the result of the determination is YES, the control returns to step S206, otherwise to step S213. Then, the newly obtained absolute values Edif 1 and 2 are compared respectively with Epre 1 and 2 indicating the Edif 1 and 2 of the prior rotation of one step. Specifically, at step S213, determination is made whether the output value of electric field sensor 10 comes nearer to or farther from the appropriate control value. According to the result of this determination, the direction of the angle control of the next step is specified (S214, S215). The procedures of steps S206-S217 are sequentially executed for each rotation of turntable 72 to maintain the appropriate control state. According to the microwave oven of the above embodiment, the metal reflector includes a metal face which becomes mainly the capacitance component and a stub which becomes mainly the induction component, and the position of the stub can be changed, whereby impedance at the heating chamber side can be changed as desired. The combination of a metal face and a stub can be selected so as to prevent arcing between the metal reflector and the wall face of the waveguide and to prevent microwave leakage. According to the microwave oven of the present embodiment, impedance matching is automatically carried out corresponding to variation of the size of the food load to be heated and cooked, the position of the food load in the heating chamber, and the change in impedance at the heating chamber side according to the cooked state of the food, whereby the microwave power efficiency can be maintained as desired. Second Embodiment Referring to Figs. 25-27, a microwave oven of a second embodiment of the present invention includes a magnetron 3 having an antenna 3-1, a waveguide 92 for conducting into an oven 91 the microwave radiated from antenna 3-1, an opening cover 91-2, a metal reflector 95 for impedance matching, a motor 98 for driving metal reflector 95 via a plastic rotation shaft 96, a switch 99 for detecting the base position of the rotation of metal reflector 95, a control circuit 100 for controlling the entire microwave oven including impedance matching control, an outer box 90, a heating chamber 91, a turntable 92, a support roller 93, a turntable motor 94, a piezoelectric weight sensor 89, and a door (not shown) attached to the opening of heating chamber 91 to open and close. Metal reflector 95 is disposed within waveguide 92, and cannot be seen from the heating chamber 91 side owing to opening cover 91-2. Magnetron 3 provided in outer box 90 radiates a microwave into waveguide 92 from antenna 3-1. The microwave radiated from antenna 3-1 passes through waveguide 92 as a traveling wave to be radiated through opening cover 91-2 to the interior of heating chamber 91. The microwave reaches the food load and is absorbed. A portion of the microwave returns to the interior of waveguide 92 according to the state of the food load. The mode of the standing wave within waveguide changes according to the ratio of the reflected wave to the standing wave and the state of its phase to affect the operation efficiency of magnetron 3. The present invention is directed to maintain highly the operating efficiency of magnetron 3 by adjusting the impedance by rotating metal reflector 95 in view of the phenomenon that impedance of the heating chamber 91 side seen from the magnetron 3 side is influenced by the state of food load. Particularly referring to Figs. 26 and 27, metal reflector 95 includes a disk 95a of a non-magnetic metal such as aluminum, and a metal stub 95a formed in a bent manner at the peripheral edge of disk 95a, which is fixed to a rotation shaft 96 formed of a low dielectric constant dielectric material such as plastic or ceramics. Rotation shaft 96 is provided at an inclining angle of α with respect to the direction of travel of the microwave, as shown in Fig. 28. Rotation shaft 96 is connected to motor 98 outside the bottom of waveguide 92 having an inclining bottom 90a corresponding to the inclining angle of α. Rotation shaft 96 is driven by motor 98. Metal stub 95b of metal reflector 95 is provided on disk 95a at an inclining angle of α with respect to disk 95a. Metal stub 95b is orthogonal to the direction of travel of the microwave when positioned at the opening cover 4 side as shown in Fig. 26. When located at the magnetron antenna 3-1 side, stub 95b is set to have an inclining angle of 2α-90° as shown in Fig. 28 with respect to the direction of travel of the microwave. Thus, the direction of the electric field (the direction of height) and the longitudinal component of L = Ls x Sin (2α-90°) of metal stub 95b show a great change. Ls is the length of metal stub 95b. Thus, the impedance adjusting width (degree of adjustment) is great. In the present embodiment, the inclining angle α of rotation shaft 96 and metal stub 95b is 65°, the diameter of disk 95a is 30mm, and the height of metal stub 95b is 18mm. By inclining rotation shaft 96 and metal stub 95b by 65°, the diameter of disk 95a can be reduced by 8mm to realize impedance matching of an equal degree in comparison with the case where rotation shaft 96 and metal stub 95b are not inclined. Therefore, the metal reflector 95 can be made more compact. As shown in Fig. 29, the portion of magnetron antenna 3-1 may be made a rectangular waveguide, and only waveguide wall 92a where metal reflector 95 and motor 98 are attached can be provided in inclined manner. The inclination angle α of rotation shaft 96 and metal stub 95b can be set to a great angle without increasing the entire size of the waveguide. The impedance adjustment width can be further increased and reflector 95 made more compact. By using a synchronous motor of a rotation velocity of 15rpm having the direction of rotation restricted to one direction for motor 98 to rotate metal reflector 95, there is a merit of reducing the cost in comparison with the case where a stepping motor or a direct current motor is used. Referring to Figs. 26-28, cam 97 for detecting the base position of rotation for controlling the rotation position is provided at the lower end of rotation shaft 96. The base position is detected by a detection switch 96 formed of a microswitch. Referring to Fig. 27, particularly, when the position of stub 95a of metal reflector 95 by the rotation of motor 98 comes to the base position for controlling the rotation position, projection 97a provided in cam 97 operates detection switch 99, whereby detection switch 99 provides an ON/OFF signal to control circuit 100. Referring to Fig. 25, control circuit 100 includes a peak hold circuit 111 for obtaining food weight information from piezoelectric weight sensor 110 which will be described later, an A/D converter 112, a logic operation control circuit 113 for controlling the rotation position of metal reflector 95, a memory 114, a timing circuit 115, and a motor driving circuit 116. The weight and position information of the food item obtained by piezoelectric weight sensor 110 are converted into digital data by A/D converter 112. According to this digital data, logic operation control circuit 113 reads out the rotation position data of metal reflector 95 from memory 114 to control the energizing time of motor 98 to stop metal reflector 95 at a predetermined position. The rotation position data of metal reflector 95 is related with load weight (weight of food) obtained by experimentally in advance as shown in Fig. 37 to be stored in memory 114 in a tabular manner. The value of the rotation position of metal stub shown in Fig. 37 represents a position where one rotation is divided into 16 (0-15). The position relation between the rotation position indicated by each numeric value and antenna 3-1 is as shown in Fig. 34. Metal reflector 95 is located at rotation position 10 in Fig. 34. Two conditions are prepared; when load is placed at the center of turntable 92; and when load is placed at the peripheral edge portion of turntable 92. Control of the rotation position of metal reflector 95 is carried out as follows. Referring to Fig. 27, cam 97 rotates according to a rotation of rotation shaft 96, whereby projection 97a operates detection switch 9. Here, time count is initiated for control of the rotation position. Since a motor is employed which takes four seconds for one rotation, metal reflector 95 is rotated by 1/16 over an energizing time period of 250msec. Fig. 28 shows the effect of metal stub 95a of metal reflector 95 towards the electric field plane (E plane) of a microwave. It can be appreciated from Fig. 28 that there is a great voltage gradient in the vertical direction because the potential of the microwave is high at the center portion and low at the circumferential portion. When metal stub 95b is at the side of opening cover 11-2 in waveguide 92, the height Ls of metal stub 95b affects the voltage gradient of the electric field of the microwave. When metal stub 95b is at the antenna 3-1 side, the component in the height direction L = Ls x Sin (2α - 90°) of metal stub 95b affects the gradient. By rotating metal reflector 95, the electric field of the standing wave in the direction of travel of the microwave is affected to result in improvement of impedance adjustment. As shown in Fig. 25, a turntable 92, and a support roller base 93 for supporting turntable 92 are provided in oven 91. A turntable motor 94 is provided outside oven 91. Referring to Fig. 30, the structure is such that a detected signal of piezoelectric weight sensor 110 provided at a passing point of roller 93a of support roller base 93 is supplied to control circuit 100. Support roller base 93 is coupled to the driving shaft of turntable driving motor 94 (Fig. 25) by means of a coupling component. Three rollers 93a supported at the end portion of support roller base 93 rotate between turntable 92 and the bottom of oven 91 by the rotation of driving motor 94 to rotate turntable 92. When roller 93a passes over piezoelectric weight sensor 110, a voltage signal shown in Fig. 31A is provided from weight sensor 110. Peak hold circuit 111 in control circuit 100 holds the level of a peak voltage signal exceeding the threshold voltage level for 200msec to 300msec as shown in Fig. 31B. A/D converter 112 reads the voltage level of the held signal to provide a digitalized data to logic operation control circuit 113. The technique regarding piezoelectric weight detection has already been employed in a microwave oven by the applicant of the present invention, and is disclosed in Japanese Patent Publication No. 3-20648 and in Japanese Utility Model Publication No. 3-17148. When food load is placed at the center of turntable 92, substantially one third of the food load is equally applied to each of the three rollers 93a. The output peak hold values Va, Vb and Vc corresponding to each of the three rollers 93a of weight sensor 110 take a substantially equal value as shown in Fig. 32A. The voltage converted value Vx of the food load weight is represented by Va + Vb + Vc. When the food load is placed at the peripheral edge of turntable 92, different weight is applied to the three rollers 93a according to the position of the food load. The output peak hold values V1, Vm and Vn corresponding to the three rollers 93a of weight sensor 95 each take a different value as shown in Fig. 32B. The voltage converted value Vy of the food load weight is represented by V1 + Vm and Vn. If the food load is identical, Vx = Vy. Whether the food load is placed at the center of the turntable is determined as follows. When all of the output peak hold values V1, Vm and Vn corresponding to the three rollers 93a are all included within the ± 30% of the voltage converted value Vy of the food load weight, determination is made that the food is placed at the center of turntable 92. When at least one of output peak hold values V1, Vm and Vn corresponding to the three rollers 93a is not included within 30% of the voltage converted value, determination is made that food is placed at the peripheral edge portion of turntable 92. That is to say, determination is made that food is placed at the center of turntable 92 if all the conditions of: Vy x 0.7 < V1 x 3 < Vy x 1.3 Vy x 0.7 < Vm x 3 < Vy x 1.3 Vy x 0.7 < Vn x 3 < Vy x 1.3 are satisfied, and determination is made that food is placed at the peripheral edge of turntable 92 if any of the above three conditions is not satisfied. Fig. 33 is a graph showing the relationship of the weight of the food load placed on turntable 92 and the output voltage of piezoelectric weight sensor 110. It can be appreciated from Fig. 33 that they are in a proportional relation. The output voltage at a no-load state where food is not placed on turntable 92 is Vw 0 , and the output voltage when a food item of 1kg is placed on turntable 92 is Vw 1 . Food weight g is obtained by the following equation: g = 1000 x (Vx - Vw 0 )/(Vw 1 - Vw 0 ) Referring to Fig. 35, curve a shows the impedance at the oven side when water load of 300cc is placed at the center of turntable 92, and curve b shows the impedance where water load of 2000cc is placed at the center of turntable 92. If the structure of waveguide 92 and the position of antenna 3-1 is symmetrical about the center line of the direction of travel of the microwave in the horizontal cross section of waveguide 92, the impedance changing locus (plot line diagram) indicated by predetermined positions of number 4-12 on the Smith chart of Fig. 13 substantially coincides with the locus from the predetermined position of number 12 to number 4 through the base position (number 0) when metal reflector makes one rotation. Therefore, position control of metal reflector 95 may be carried out by either locus. However, the waveguide may not be symmetrical. Therefore, the position control of metal reflector 95 in the present embodiment is carried out within the rotation range of predetermined position numbers 4-12 to improve the accuracy of position control. In the Smith chart of Fig. 35, the impedance for magnetron 3 to operate effectively and the maximum output operation region of magnetron 3 coincide with each other. Therefore, a microwave oven of high power efficiency can be realized by adjusting the impedance of the oven side to exist within this maximum output region according to the food load condition in the oven. Thus, according to the Smith chart of Fig. 35, the rotation position of metal reflector 95 is preferably set to number 9-11 on curve a, and to number 7-10 on curve b in the case of a light load of approximately 300cc and in the case of a heavy load of approximately 2000cc, respectively. Fig. 36 shows a high frequency output in the case of loads of 2000cc and 300cc under parameters of the rotation position of metal stub 95b (metal reflector 95). Referring to Fig. 36, the rotation position of a high output state substantially coincides with the optimum rotation position of metal reflector 95 by the Smith chart of Fig. 36. In the graph of Fig. 36, the high frequency output when the food load is placed at the center of turntable is shown by a block dot and a solid line, and the high frequency output when the food load is placed at the peripheral edge of the turntable is shown by a white circle and a broken line. If the load is light, the case must be considered where a food item small in size is placed at the peripheral edge of turntable 72. If the food load is great such as that exceeding 1.5kg in weight, the container is also great in size and will naturally be placed at the center portion of the turntable. Therefore, in this case, the condition of load placed at the peripheral edge does not have to be considered. According to the graph of Fig. 36, the rotation position of metal reflector 95 in the case of a food load corresponding to water of 2000cc is preferably at number 8. In the case of a food load corresponding to water of 300cc, the rotation position of metal reflector 95 is preferably at number 10 and at number 8 when food is placed at the center and at the peripheral edge portion of turntable 92, respectively. Fig. 37 is a graph of the rotation angle of metal stub 95b (metal reflector 95) obtained by measuring data shown in Fig. 36 under the parameter of other load weight. The rotation position relation of metal reflector 95 corresponding to the load weight indicated in the graph is stored in advance as a data table in memory 114 of control circuit 100. Logic operation control circuit 113 looks up the rotation position of metal reflector 95 corresponding to a load weight from the data table for usage of control. In heating light food of load not more than 100g in weight (including no-load operation), a great thermal stress is applied in the microwave oven. Because a great power of microwave is applied into the oven that has scarcely any load, local heating in turntable 92, support roller base 93 and the wall face of oven 91, and heat up of magnetron 3 due to reflected power of the microwave are generated. This problem is alleviated to some degree according to the control of the rotation position of metal reflector 95. However in such a case, there is no load for greatly consuming the microwave, for example, a great food load, and this control of rotation position is critical. Furthermore, considering variation in the components produced by mass production, it is impossible to correspond to all the products of mass production. Thus, a completely different method of rotating continuously metal reflector 95 at the time of operation of a light load not more than 100g is employed. Rotation of metal reflector 95 will shift the local heated position to disperse the local heat. No Load - 100g 101-200g 201-500g 501-1500g 1501-3000g More Than 3000g A B A B Warming Rotate 12 11 10 9 7 6 5 Milk Rotate 12 12 10 10 7 8 7 Warming Sake Rotate 12 11 8 9 6 7 4 Vegetable Preparing Rotate 12 12 9 8 7 6 4 Rice Cooking Rotate 12 10 8 9 7 6 5 Defrosting Rotate Rotate Rotate Rotate Rotate Rotate Rotate Rotate Manual Cooking Rotate 12 10 8 9 7 8 7 A: load placed at center B: load placed at peripheral edge Rotate: Rotate metal reflector during heating operation Table 1 is a data table for controlling the rotation of the metal reflector corresponding to a cooking menu used in the microwave oven of the present embodiment. Because the impedance at the oven side is different even with the same load weight depending on to the shape of the food and the container according to the cooking menu, impedance matching can be controlled more appropriately by the rotation control data of metal reflector 95 for each cooking menu. Regarding defrosted cooking operation, attention is directed to the following points. There is difference in dielectric constant with respect to microwave according to a portion not yet defrosted and the defrosted portion of the frozen food. This difference in dielectric constant causes only the defrosted portion to be easily heated due to concentration of an electric field in the defrosted portion. It is necessary to avoid concentration of microwave electric field in the food item during defrost cooking operation. For this purpose, metal reflector 95 is rotated continuously. The local concentration portion of microwave can be dispersed to prevent local heating, so that high microwave power can be provided. Thus, the time required for defrosting can be significantly reduced. The position control of a metal reflector starting from initiation of a heating operation until the end of heating is described hereinafter. In order to identify the food load, support roller base 93 (roller 93a) of turntable 92 must pass over weight sensor 110 at least three times. Therefore, at the start of a heating operation, the weight of load is not yet detected and the control data of the reflector shown in Fig. 37 and Table 1 cannot be utilized. Thus the position of metal reflector 95 is set presuming that a food item of light load is cooked until a control based on a detected weight can be carried out. More specifically, the initial position of metal reflector 95 is set to the position of number 10 as shown in Fig. 34 in the present embodiment. Thus, the cooking time of a food of light load can always be reduced. According to the three input information from weight sensor 110 according to a rotation of support roller base 93, the weight of the food item is calculated and determination is made whether the food is placed at the center portion or at the peripheral edge of turntable 92 as described above. From the data table corresponding to the determined position of the food item (the data table relating to the rotation position of the metal reflector with respect to food weight), the rotation position of reflector 95 is looked up corresponding to the calculated weight. Motor 98 is rotatably driven to rotate metal reflector 95 to the obtained position from the initial position. This control of position has been described in the foregoing with reference to Fig. 27. Because metal reflector 95 passes the base position for every position setting in this position control, the stop position is very precise even if there is problem in the accuracy of the timing of initiating the rotation of motor 98. This heating of high power efficiency with impedance matched is carried out until elapse of a specified heated time or until the sensor detects completion of cooking. If metal reflector 95 is set at an initial position corresponding to a light load food, heating of light load food can be carried out by high power efficiency from the start of a next heating cooking operation. According to the present embodiment, the metal reflector for matching impedance can be made compact to avoid arcing phenomenon with the wall of the waveguide. A microwave oven of low cost can be provided that carries out heating of high power efficiency according to the weight and placed position of a food item be heated or according to a cooking menu. Furthermore, the problem of local heating encountered an the time of operation of a light load including no load and at the time of defrost cooking can be solved. As a result, the microwave power at the time of defrost cooking can be improved accordingly to carry out defrosting uniformly to reduce the defrosting time.

Claims: A microwave oven comprising: a heating chamber (1, 91) having an opening through which a microwave is introduced therein, means (3) for generating a microwave, a waveguide (52, 92) for guiding said microwave in a predetermined direction of travel to introduce the same into said heating chamber (1, 91) via said opening, means (55, 55a-c, 95) provided rotatably about a predetermined shaft (56, 96) in the proximity of said opening in said waveguide (52, 92) for adjusting impedance of load at said heating chamber (1, 91) side by a rotation angle thereof, and means (57-59, 68, 89, 92-94, 97-100) for controlling a rotation angle of said means (55) for adjusting impedance, wherein said means (55, 55a-c, 95) for adjusting impedance comprises a metal reflector (55, 55a-c, 95) characterized by said rotation shaft (56, 96) provided in said waveguide (52, 92) so as to cross said direction of travel by a predetermined angle α, said refletor comprising a first member (81, 83, 88, 95a) having a metal face attached to said rotation shaft (56, 96), and a second member (82, 84, 86, 95b) formed projecting from said metal face at a prescribed angle α at a position in said waveguide (52, 92), said position moving in accordance with rotation of said rotation shaft (56, 96) on said metal face. The microwave oven according to claim 1, wherein said rotation shaft (56, 96) is formed of a dielectric. The microwave oven according to claim 2, further comprising means (10) provided at the inner wall face of said waveguide (52) or said heating chamber (1) for detecting an electric field, wherein said means (57-59, 68, 89, 92-94, 97-100) for controlling a rotation angle of said means (55) for adjusting impedance comprises means (57-59, 68, 89, 92-94, 97-100) responsive to an output of said electric field detecting means (10) for controlling a rotation angle of said means for adjusting impedance, and said rotation shaft (56) is provided substantially perpendicular to the direction of travel of said microwave. The microwave oven according to claim 3, wherein said first member (81, 83, 88) has said metal face attached to said rotation shaft (56) so as to be substantially parallel to said direction of travel. The microwave oven according to claim 4, wherein said second member (82, 84, 86) is formed substantially at right angles with said metal face. The microwave oven according to claim 2, wherein said predetermined angle α is selected to be greater than 0° and smaller than 90° toward said heating chamber (91). The microwave oven according to claim 6, wherein said second member (95b) is provided so that its center axis is inclined by said predetermined angle α at toward said rotation shaft side with respect to said metal face. The microwave oven according to claim 3, wherein said control means (57-59, 68) comprises means (57-59, 68) for adjusting the rotation angle of said means (55, 55a-c) for controlling impedance so that the output of said means (10) for detecting intensity of an electric field is reduced. The microwave oven according to claim 3, further comprising a turntable (72) provided in said heating chamber (1) for supporting a substance to be heated. The microwave oven according to claim 9, wherein said control means comprises means (64) for prestoring intensity of a target electric field prepared by a predetermined method, and means (57-59, 62, 63, 65, 66) for controlling the rotation angle of said metal reflector (55) so that the output of said means (10) for detecting intensity of an electric field and said intensity of the target electric field have a predetermined relationship. The microwave oven according to claim 10, wherein said means for controlling the rotation angle of said metal reflector comprises means (62, 63, S206-S210, S206-S210A) for obtaining a predetermined representative value of an output of said means (10) for detecting intensity of an electric field over a predetermined number of rotations of said turntable (72), means (S211, S211A) for obtaining difference between said representative value and said target value, and means (S212-S216) for adjusting the rotation angle of said metal reflector (55, 55a-55c) so that said difference between said representative value and said target value is within a predetermined range. The microwave oven according to claim 11, wherein said representative value is a maximum value. The microwave oven according to claim 11, wherein said representative value is a minimum value. The microwave oven according to claim 11, wherein said predetermined number is one. The microwave oven according to claim 11, wherein said means (S206-S210A) for obtaining said predetermined representative value comprises means (S206-S210A) for obtaining a predetermined plurality of representative values of an output of said means (10) for detecting intensity of an electric field over a predetermined number of rotations of said turntable (72). The microwave oven according to claim 15, wherein said plurality of representative values comprise a maximum value and a minimum value. The microwave oven according to claim 3, wherein said control means comprises first means (S201-S204, S207-S209) for controlling the rotation angle of said means (55, 55a-c) for adjusting impedance according to a first control method during initiation of operation of said microwave oven until a predetermined condition is met, second means (S210-S217) for controlling the rotation angle of said means (55, 55a-c) for adjusting impedance according to a second control method different from said first control method after said predetermined condition is met following initiation of operation of said microwave oven. The microwave oven according to claim 17, wherein said first means comprises means (S202-S204) for fixing the rotation angle of said means (55, 55a-c) for adjusting impedance so that, when a predetermined load is placed at the center of the bottom portion of said heating chamber (1), impedance of said predetermined load is in a position where said microwave generating means (3) generates a maximum output. The microwave oven according to claim 18, wherein said second means comprises means (64) for prestoring a target rotation angle of said means (55, 55a-c) for adjusting impedance prepared in advance corresponding to different weight of a substance to be heated, means (64) for prestoring a target value of an output of said means (10) for detecting intensity of an electric field, means (73, 75) for detecting weight of a substance to be heated conveyed into said heating chamber (1), and first control means (S212-S216) for controlling the rotation angle of said means (55, 55a-c) for adjusting impedance so that output of said means (10) for detecting intensity of an electric field approximates the target value with the target rotation angle of said means (55, 55a-c) for adjusting impedance corresponding to said detected weight set as an initial value. The microwave oven according to claim 19, further comprising a turntable (72) provided in said heating chamber (1) for supporting a substance to be heated. A microwave oven according to claim 20, wherein said first control means for controlling the rotation angle comprises second control means for controlling the rotation angle of said means (55, 55a-c) for adjusting impedance so that a predetermined representative value of an output of said means (10) for detecting intensity of said electric field over a predetermined number of times of rotation of said turntable (72) and said target value have a predetermined relationship. The microwave oven according to claim 21, wherein said second means for controlling the rotation angle of said means for adjusting impedance comprises means (S206-S210, S210A) for obtaining a predetermined representative value of an output of said means (10) for detecting intensity of an electric field over a predetermined number of times of rotation of said turntable (72), means (S211, S211A) for obtaining difference between said representative value and said target value, and means (S212-S217) for adjusting the rotation angle of said means (55, 55a-c) for adjusting impedance so that said difference between said representative value and said target value is within a predetermined range. The microwave oven according to claim 22, wherein said representative value is an average value. The microwave oven according to claim 23, wherein said predetermined number is one. The microwave oven according to claim 24, wherein said means for obtaining said predetermined representative value comprises a support roller (73) for supporting said turntable (72) by a bottom portion thereof and for moving in a circular motion on the bottom of said heating chamber (1) in accordance with rotation of said turntable (72), a weight sensor (75) provided at a passing position of said support roller (73) for generating a predetermined detection signal every time said support roller (73) passes over said passing position, sampling means (62, 63, 65, S206) for sampling an output of said means (10) for detecting intensity of an electric field at a predetermined sampling frequency, means (S209) responsive to said detection signal for initiating integration of said sampling means, and responsive to a next said detection means for providing an integrated value, and means (S210) for dividing said integrated value by a numeric value determined by said sampling frequency for providing an average intensity of said means (10) for detecting intensity of an electric field. The microwave oven according to claim 1, wherein said control means comprises means (14) for prestoring a table prepared in advance of target rotation angles of said means (95) for adjusting impedance corresponding to different weight of a substance to be heated, means (89) for detecting weight of a substance to be heated conveyed into said heating chamber (91), and means (100) for controlling the rotation angle of said means (95) for adjusting impedance by collating said detected weight with said table of target rotation angles. The microwave oven according to claim 26, wherein said control means further comprises means for detecting no load in said heating chamber (91), and means responsive to detection of no load for rotating continuously at a predetermined speed said means (95) for adjusting impedance.

Inventors: FUKUI MASATUGU, KINOSHITA HIROSHI, MINAKAWA HIROSHI, UNO MASAYUKI, FUKUI, MASATUGU, KINOSHITA, HIROSHI, MINAKAWA, HIROSHI, UNO, MASAYUKI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1993
Main classification: H05B6
IPC codes: H05B6