High-frequency heating apparatus

Abstract: A high-frequency heating apparatus (K) comprising: a high-frequency oscillator (1) for oscillating high-frequency electromagnetic wave by electric power supplied from a power source circuit (11); a heating chamber (3) into which the high-frequency electromagnetic wave is supplied by the high-frequency oscillator (1); an antenna (7) which is provided outside the heating chamber (3) and adjacent to an opening (6) of the heating chamber (3); a dielectric plate (5) for covering the opening (6), which is provided in the heating chamber (3) so as to confront the antenna (7) through the opening (6); a detector (8) which receives an output from the antenna (7) and has a grounded portion (15, 16) such that the grounded portion (15, 16) is connected to the heating chamber (3); and a control circuit (10) which receives an output from the detector (8) so as to output a control signal to the power source circuit (11).

Description: The present invention relates to a high-frequency heating apparatus such as an electronic range, in which a high-frequency heat source, e.g. a magnetron is controlled by detecting field intensity in a cabinet. A high-frequency heating apparatus is known from, for example, JP-A-59-207595 in which by using transmitting and receiving antennas confronting a heating chamber, changes of dielectric constant of an article to be heated (hereinbelow, referred to as "food") dependent upon temperature of the food are detected so as to control a high-frequency heat source. However, the known high-frequency heating apparatus in which the antennas confront the heating chamber has such a drawback that especially at the time of heating of the food, a large amount of water or oil from the food scatters in the cabinet and penetrate into a contact point between the receiving antenna and a detector, thereby resulting in great change in detection characteristics. From EP-A-461 269, to be considered under Article 54(3) EPC, a high frequency heating device is already known in which an antenna for receiving part of electromagnetic waves radiated from a radiowave radiating part into a heating room and a detector for detecting electric power received by the antenna are provided together on a printed circuit board. Operation of the high frequency heating device is controlled by a controller based on the output of the detector to optimally cook food. It is an object of the present invention to provide a high frequency heating apparatus in which an antenna is provided outside a heating chamber so as not to be contaminated by water or oil scattered from food in the heating chamber. According to the invention, a high frequency heating apparatus is provided, comprising a high-frequency oscillator for oscillating high-frequency electromagnetic wave by electric power supplied from a power source circuit, a cabinet having a shape of rectangular parallelepiped and surrounding a heating chamber into which the high-frequency electromagnetic wave is supplied by said high-frequency oscillator, an antenna which is provided outside said heating chamber and adjacent to an elongated opening in a wall of the cabinet, a dielectric plate for covering the opening, which is provided in said wall of said cabinet so as to confront said antenna through the opening, a detector which receives an output from said antenna and has a grounded portion such that said grounded portion is connected to said cabinet, and a control circuit which receives an output from said detector so as to output a control signal to said power source circuit, said cabinet defining, in a plane at which the opening confronts said dielectric plate, a contour having a straight wall portion and said elongated opening extending with its longitudinal direction obliquely relative to the straight wall portion of the contour. Preferable embodiments are defined in the dependent claims. Features of the present invention will become apparent from the following description taken in conjunction with preferred embodiment thereof with reference to the accompanying drawings, in which: Fig. 1 is a schematic view of a high-frequency heating apparatus according to one embodiment of the present invention; Fig. 2 is a fragmentary sectional view of the heating apparatus of Fig. 1; Figs. 3a, 3b and 3c are views observed in the directions of the arrows IIIa-IIIa, IIIb-IIIb and IIIc-IIIc in Fig. 2, respectively; Fig. 4 is a graph showing temperature characteristics of dielectric loss of a food in the heating apparatus of Fig. 1; Fig. 5 is a graph showing wave form of detection output in the heating apparatus of Fig. 1; and Figs. 6a and 6b are views similar to Fig. 3c, particularly showing first and second modifications thereof, respectively. Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerals throughout the accompanying drawings. Referring now to the drawings, there is shown in Fig. 1, a high-frequency heating apparatus K according to one embodiment of the present invention. In the heating apparatus K, electromagnetic wave emitted from a high-frequency oscillator 1 is supplied, through a waveguide 2, into a high-frequency heating chamber 3 so as to heat a food 4 in a cabinet 30 having a shape of rectangular parallelepiped. Electromagnetic wave in the cabinet 30 is detected, via a dielectric plate 5 and an opening 6 of the cabinet 30, in direct current by a detector 8 provided with an antenna 7. The detector 8 has a grounded conductor whose one portion is connected to a wall of the cabinet 30. A current signal detected by the detector 8 is fed, through an amplifier 9, to a control circuit 10 leading to a power source circuit 11. Since the amplifier 9 is provided between the detector 8 and the control circuit 10, the power source circuit 11 can be controlled stably at a high signal level against noises. Fig. 2 and Figs. 3a to 3c show the opening 6 and the detector 8. By using machine screws 13, the detector 8 is secured to a bracket 12 attached to an outer surface of the wall of the cabinet 30. The detector 8 is formed by a microstrip line including an active conductor 14 and grounded faces 15 and 16. The detector 8 further includes resistors 17, 18 and 19, a diode 20 and a capacitor 21. The grounded faces 15 and 16 are connected to each other by forming a through-hole or by a connecting conductor 22. Since the grounded face 16 is held in contact with the bracket 12, the grounded faces 15 and 16 of the microstrip line have a potential identical with that of the heating chamber 3, so that a microwave transmission circuit functioning stably is obtained. By using another connecting conductor 23, a conductor piece at the side of the grounded face 16 is connected to the active conductor 14 at the side of the grounded face 15 so as to act as the antenna 7. The dielectric plate 5 is fixed to an inner surface of the wall of the cabinet 30 by bonding agent, etc. so as to cover the opening 6. Therefore, the dielectric plate 5 confronts the antenna 7 through the opening 6 and prevents water and oil in the cabinet 30 from reaching the antenna 7 directly. Lead wires 24 and 25 are, respectively, attached to the active conductor 14 and the grounded face 15 by solder, etc. and are led to the amplifier 9. The opening 6 is of a crossed shape having crossing portions 6a and 6b and the crossing portions 6a and 6b are inclined at an angle  relative to a horizontal direction of the cabinet 30 as shown in Fig. 3c. Meanwhile, as shown in Fig. 2, the heating chamber 3 defines a rectangular contour having a straight portion 30A, etc. in a plane at which the opening 6 confronts the dielectric plate 5. Therefore, the crossing portions 6a and 6b extend obliquely relative to the straight portion 30A of the contour and thus, the antenna 7 is least likely to be affected by mode change of standing wave in the heating chamber 3. As a result, average whole change of dielectric loss in the heating chamber 3 can be received by the single antenna 7 without the need for providing a plurality of the antennas. Meanwhile, since the crossing portions 6a and 6b deviate from a longitudinal direction of the antenna 7 as shown in Figs. 3b and 3c, average whole change of dielectric loss in the heating chamber 3 can be received by the antenna 7. Meanwhile, in the above embodiment, the opening 6 has a crossed shape. However, the opening 6 is not restricted to the crossed shape but may have any elongated shape such as an opening 6' in Fig. 6a or an opening 6'' shown in Fig. 6b such that a longitudinal direction of the opening 6' or 6'' extends obliquely relative to the straight portion 30A of the contour. Likewise, the longitudinal direction of the opening 6' or 6'' deviates from the longitudinal direction of the antenna 7. Furthermore, in the above embodiment, the opening 6 is formed in the side wall of the cabinet 30. However, the present invention can also be applied to an arrangement in which the opening 6 is formed in the top plate of the cabinet 30. Fig. 4 shows temperature characteristics of dielectric loss (ε r x tanδ) of beef or fish measured at a frequency of 2,400 MHz in the heating apparatus K. It is apparent from Fig. 4 that dielectric loss changes greatly among a frozen state, a defrozen state, a room temperature state and a heated state of the food. This phenomenon in which dielectric loss is great indicates that electromagnetic wave is well absorbed by the food. Fig. 5 shows one example of detection output in the case of heating beef from a frozen state in the heating apparatus K. From Figs. 4 and 5, it will be seen that when dielectric loss of the food is small, detection output is large. On the other hand, when dielectric loss of the food is large, detection output becomes small. Therefore, by controlling the power source circuit 11 on the basis of magnitude of detection output or trend of change of detection output, it becomes possible to automatically detect defreezing or heating of the food. As is clear from the foregoing, in the heating apparatus of the present invention, the antenna is provided outside the heating chamber and electromagnetic wave from the opening of the cabinet is received through the dielectric plate so as to be detected. Furthermore, the grounded faces of the detector are connected to the heating chamber. Therefore, in accordance with the present invention, even if water or oil scatters from the food, such an undesirable phenomenon does not take place that the antenna is short-circuited to the grounded faces by water or oil of the food, so that stable control performance of the heating apparatus can be secured for a long term. Moreover, even if mass production of the heating apparatus is performed, the detector can function stably. Meanwhile, since the conductor piece of the printed circuit board which constitutes the detector formed by the microstrip line acts as the antenna, dimensional accuracy of the antenna is more excellent than an arrangement in which an antenna is provided outwardly of the printed circuit board or an arrangement in which a metallic rod acting as an antenna is vertically erected on the printed circuit board. Therefore, in accordance with the present invention, the antenna has stable microwave characteristics. In addition, by using a frequency filter circuit based on the microstrip line constituted by the printed circuit board, electric parts for the detector such as the resistors, the diode and the capacitor may function at a relatively low frequency, so that the detector can be produced at low cost and stably. Meanwhile, since the longitudinal direction of the opening extends obliquely relative to the straight portion of the contour defined by the heating chamber in the plane at which the opening confronts the dielectric plate, the antenna is least likely to be affected by mode change of standing wave in the heating chamber. Therefore, in the detector of the present invention, average whole change of dielectric loss in the heating chamber can be received by the single antenna without the need for providing a plurality of the antennas. Furthermore, since the longitudinal direction of the opening deviates from the longitudinal direction of the antenna, average whole change of dielectric loss in the heating chamber can be received by the antenna. Moreover, since the amplifier is provided between the detector and the control circuit, the power source circuit can be controlled at a high signal level against noises. Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be noted here that various changes and modifications will be apparent to those skilled in the art within the scope of the appended claims.

Claims: A high-frequency heating apparatus (K) comprising: a high-frequency oscillator (1) ) for oscillating high-frequency electromagnetic wave by electric power supplied from a power source circuit (11); a cabinet (30) having a shape of rectangular parallelepiped and surrounding a heating chamber (3) into which the high-frequency electromagnetic wave is supplied by said high-frequency oscillator (1); an antenna (7) which is provided outside said heating chamber (3) and adjacent to an elongated opening (6a, 6b, 6', 6") in a wall of the cabinet (30); a dielectric plate (5) for covering the opening (6a, 6b, 6', 6"), which is provided in said wall of said cabinet (30) so as to confront said antenna (7) through the opening; a detector (8) which receives an output from said antenna (7) and has a grounded portion (15, 16) such that said grounded portion (15, 16) is connected to said cabinet (30); and a control circuit (10) which receives an output from said detector (8) so as to output a control signal to said power source circuit (11); said cabinet (30) defining, in a plane at which the opening (6a, 6b, 6', 6") confronts said dielectric plate (5), a contour having a straight wall portion (30A) and said elongated opening (6a, 6b, 6', 6") extending with its longitudinal direction obliquely relative to the straight wall portion (30A) of the contour. A high-frequency heating apparatus as claimed in claim 1, comprising two elongated openings (6a, 6b) together forming a cross shape. A high-frequency heating apparatus as claimed in claim 1, wherein the opening (6', 6") is an elongated slot being oblique to the straight wall portion (30A) by an angle (Θ). A high-frequency heating apparatus (K) as claimed in any of claims 1-3, wherein said detector (8) is formed by a printed circuit board such that a conductor piece of said printed circuit board acts as said antenna (7). A high-frequency heating apparatus (K) as claimed in any of claims 1-4, wherein the longitudinal direction/directions of the opening (6', 6")/openings (6a, 6b) deviates/deviate from a longitudinal direction of said antenna (7). A high-frequency heating apparatus (K) as claimed in any of claims 1-5, further comprising: an amplifier (9) which is provided between said detector (8) and said control circuit (10).

Inventors: KASHIMOTO TAKASHI, KUSUNOKI SHIGERU, YOSHINO KOJI, KASHIMOTO, TAKASHI, KUSUNOKI, SHIGERU, YOSHINO, KOJI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1991
Main classification: H05B6
IPC codes: H05B6, F24C7