Title:
ENERGY SEAL FOR MICROWAVE OVEN APPARATUS
United States Patent 3584177


Abstract:
An oven apparatus is disclosed utilizing electromagnetic energy at microwave frequencies for heating by radiation within an electrically conductive enclosure having an access opening closed by a substantially flush-mounted door arrangement. To substantially reduce the radiation of such energy through the door edges during operation an energy seal is provided along the peripheral walls of the opening comprising a short-circuited one-quarter wavelength-type choke structure. Such a structure is provided with a layer of conductive material adjacent to the wall surfaces to form substantially a double wall and supporting the high leakage currents as a result of the escaping radiated energy to thereby enhance the efficiency of the choke under all operating conditions.



Inventors:
BUCKSBAUM ARNOLD M
Application Number:
04/869106
Publication Date:
06/08/1971
Filing Date:
10/24/1969
Assignee:
AMANA REFRIGERATION INC.
Primary Class:
International Classes:
H05B6/76; (IPC1-7): H05B9/06; H05B5/00
Field of Search:
219/10.55
View Patent Images:
US Patent References:
3210512Ovens1965-10-05Eason
3196242High-frequency oven door seal1965-07-20De Vries et al.
3182164Electromagnetic energy seal1965-05-04Ironfield
2958754Electronic ovens1960-11-01Hahn



Primary Examiner:
Staubly R. F.
Assistant Examiner:
Bender L. H.
Claims:
What I claim is

1. In combination:

2. In combination:

3. In combination:

4. Microwave oven apparatus comprising:

5. Microwave oven apparatus comprising:

6. Microwave oven apparatus according to claim 5 wherein said choke arrangement is disposed along the peripheral edges of said door.

7. Microwave oven apparatus according to claim 5 wherein said choke arrangement is disposed along the peripheral oven walls around said opening;

8. Microwave oven apparatus according to claim 5 wherein said conductive layer covers a substantial portion of the perimeter walls defining said choke arrangement.

9. Microwave oven apparatus according to claim 5 wherein said conductive layer is supported within said choke arrangement by a body of a dielectric material.

10. Microwave oven apparatus according to claim 5 wherein said choke arrangement is filled with a dielectric material with said conductive material coated on the dielectric material in the areas contacting the inner choke wall surfaces.

11. In combination:

12. In combination:

13. High frequency heating apparatus adapted to be supplied with electromagnetic wave energy comprising:

14. An energy seal for ovens adapted to be radiated with electromagnetic energy comprising:

Description:
BACKGROUND OF THE INVENTION

In the microwave heating art adequate shielding to prevent escape of the radiated electromagnetic waves through the oven door edges during operation has presented a continuing problem. The electric field intensity is very high within the oven enclosure and it is necessary to minimize radio frequency interference as well as comply with the radiation safety hazard standards established, respectively, by the Federal Communications Commission and United States of America Standards Institute.

Numerous prior art solutions have included the use of conductive gaskets surrounding the access opening or other suitable metal-to-metal contacts together with spring-loaded hinged doors. Sliding doors contacting the front planar surface of the peripheral walls surrounding the access opening have also been suggested. Such contiguous metal-to-metal polar contact or gasket arrangements have inherent shortcomings since they tend to become coated and erode after extended periods of operation which may lead to severe leakage and arcing.

An improved prior art electromagnetic energy seal provides for planar contact closure areas between the access opening and peripheral walls of approximately one-quarter of a wavelength in a single overlapping door arranged similar to a radio frequency transmission choke structure. Still another suggested solution includes a choke-type energy seal having an approximate overall length of one-half wavelength and nonmetallic contact areas for a slidably mounted door. In such a structure a predetermined escape route is provided for stray radiated energy by the spaced wall surfaces which may be filled with an energy permeable dielectric medium to reduce the choke dimensions over structures in air and to enhance cleanliness. Briefly, the energy seal defines a choke structure analogous to conventional parallel plate radio frequency transmission line circuits to provide a path of least resistance for the escaping energy. A gap oriented near the frontal planar surface of the oven provides a first path of substantially one-quarter of a wavelength before the radiated energy contacts a metallic conductive surface. The energy is then reflected or transformed along a second path of one-quarter wavelength terminating in a conductive surface thereby located an overall distance of an integral number of one-half wavelengths from the point of origin of the escaping energy. The terminating conductive surface, therefore, presents a low impedance or short circuit with high surface currents. This low impedance is transformed back one-quarter wavelength as a high impedance and high voltage node with little or no currents present. In turn, this high impedance is reflected back into a short circuit at the point of origin. This short circuit then, substantially prevents the leakage of the electromagnetic energy radiation from the oven into the environment Further, complete details regarding the door choke type energy seal may be had by referring to U.S. Pat. No. 3,182,164 issued May 4, 1965 to Richard Ironfield and assigned to the Raytheon Company.

The prior art electromagnetic energy seals, particularly those of the radio frequency choke type, require the use of costly highly electrical conductive metallic components for the doors and wall surfaces of the access opening to support the high surface currents at the termining conductive surface. In addition, costly metallurgical bonding techniques, such as spot welding, are required in the fabrication of the pertinent structures. After operation, the efficiency of the foregoing choke arrangements which are critically dependent on dimensions for surface current carrying capabilities may be sharply diminished or altered. At the point where the low impedance or short circuit characteristics are desired, the terminating conductive surface may reflect in reality, a high impedance to the point of origin of the choke adjacent to the oven enclosure. High electromagnetic energy radiation leakage would, therefore, follow upon operation at levels possibly above the maximum exposure level consonant with interference and safety standards.

For the purposes of the present specification, the term "microwaves" shall refer to electromagnetic wave energy radiation in that portion of the spectrum from approximately 1,000 to 300,000 megacycles per second, corresponding to wavelengths of from 30 centimeters to 1 millimeter. The oven apparatus disclosed in the present invention utilizes a magnetron as an energy source which radiates at a frequency of substantially 2,450 megacycles per second corresponding to a wavelength of approximately 5 inches in air. Further, the expression "wave lengths" shall be utilized to refer to the wavelength of the electromagnetic energy as it travels in the medium defined by the choke arrangement at the operating frequency of the magnetron generator. Additionally, the term shall denote the effective wavelength within the choke structure filled with a dielectric medium which will result in shorter wavelengths of the radiated energy as compared with propagation only in free space.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention a microwave oven apparatus is provided with a primary energy seal arrangement having two parallel paths terminating in a conducting surface which are microwave permeable and define an overall leakage path of approximately an integral number of one-half wavelength from a point of origin. The high electrical surface current carrying wall structure along the internal perimeter of the choke arrangement is engaged by a layer of a conductive material such as a thin metallic foil. The conductivity of the choke wall structure is thereby rendered less critical in view of the fact that the high surface currents will now be carried by the conductive layer material. In the choke embodiments providing for the disposition of a dielectric medium preferably having a dielectric constant value greater than air and being substantially microwave permeable, the conductive layer may be wedged or supported by the dielectric material. The conductively lined choke arrangement may be disposed along the peripheral edges of the oven door to provide for mounting entirely within the access opening. Alternatively, the choke arrangement structure disclosed herein may be provided as a component of the oven enclosure walls adjacent to the access opening.

In the embodiments of the microwave oven apparatus disclosed herein, metallic materials for the door as well as choke arrangement may now employ mechanical rather than metallurgical bonding techniques with an accompanying savings in labor and material costs. In addition with the electrical circuitry of the choke structure preserved by means of the conductive layer, materials having poorer electrical conductivity characteristics which render them unsuitable for electrical resistance welding techniques may be employed for the oven door closure means. It is, therefore, within the purview of the invention to substitute less costly metals such as aluminum for such metals as stainless steel with attendant savings in overall apparatus costs. The conductive material layer will also enhance the efficiency of the safety structure of the overall device over extended periods of operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, as well as the details for the provision of a preferred embodiment, will be readily understood after consideration of the following detailed description and reference to the accompanying drawings, wherein:

FIG. 1 is a front perspective view of a microwave oven;

FIG. 2 is a similar perspective view of the embodiment of the invention with the door closure means opened;

FIG. 3 is a top view with a portion of the oven walls removed to reveal internal structure;

FIG. 4 is an enlarged fragmentary cross-sectional view of a prior art dielectric filled choke arrangement provided in the door closure means of an illustrative embodiment;

FIG. 5 is a diagrammatic sketch illustrative of the operating principles of the choke structure arrangement of the present invention;

FIG. 6 is a partial cross-sectional view illustrative of a door choke arrangement incorporating the present invention;

FIG. 7 is a partial cross-sectional view of an alternative embodiment of the door-type choke arrangement of the present invention;

and FIG. 8 is a partial cross-sectional view of another alternative embodiment wherein the choke arrangement is provided in the microwave oven apparatus wall surfaces surrounding the access opening.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIGS. 1 to 3 inclusive, the microwave oven apparatus 10 embodying the features of the present invention comprises a hollow oven enclosure 12 defined by rectangular parallelpiped electrically conductive walls 14 of a suitable metal. A casing 16 surrounds the oven enclosure as well as accompanying components and controls. Access to the interior is provided by opening 18 with a metallic door closure means 20 supported by spring actuated arms 22 and hinge 24 together with a handle 26 to enable manual opening and closing thereof. The door 20 comprises a panel member 28 as well as ring member 30 secured along the perimeter of the panel member. The door assembly is positioned within the access opening 18 and is flush mounted with respect to the oven walls. Decorative trim members 32 are disposed along the periphery of the door panel and ring members in the manner of a picture frame and expose perforations 34 defining approximately 150 openings per square inch covering a major portion of the door panel member to prevent the escape of electromagnetic energy radiation during the operation of the oven apparatus with the door closing the access opening.

Control panel 36 is disposed adjacent to the oven enclosure opening with the electromagnetic energy generation means and accompanying electrical circuitry disposed behind. The electromagnetic generation means, illustratively a microwave magnetron, along with the requisite power supply and controls indicated generally by box 38, feed the required energy by means of a radiating probe 40 in dielectric dome member 42 to a section of rectangular waveguide 44 having an appropriately dimensioned open end 46 within the cavity enclosure 12. The opposing end of waveguide 44 is closed as at 48 to prevent escapement of electromagnetic energy in this direction. Upon energizing of the microwave magnetron, the waves radiating within the oven enclosure are uniformly distributed by a stirrer 50 having a plurality of vane members 52 which are driven by a fractional horsepower motor. The stirrer means are well known in the art and have been described in detail in U.S. Pat. No. 2,813,185, issued Nov. 12, 1957 to Robert V. Smith.

Referring now to FIG. 4, the door-type choke arrangement 54 is illustrated. The door panel 28 defines a peripheral upstanding wall section 28a which defines with the oven enclosure wall 14 a first electromagnetic energy transmission path 56 having its point of origin at point 58 adjacent wall 14. The ring member 30 has a stepped configuration to provide a conductive wall 30a for the choke arrangement and define a frontal lateral member 30b overlapping the oven enclosure and casing walls 14 and 16, respectively. In accordance with a prior art construction, the door ring member 30 is electrically welded to the panel 28 to thereby frame perforations 34. In the fabrication of the overall door assembly then, a plurality of spot welds will be provided on four sides of the panel member 28. Ring member 30 defines with the door panel section 28a a second parallel electromagnetic energy transmission path 60. The entire choke structure may be filled with a dielectric medium which is electromagnetic energy permeable and, preferably, has a dielectric constant value of between 2 and 4. Illustratively, polystyrene or polypropylene may be utilized for the dielectric material. The complete choke arrangement comprises what is referred to herein as the "primary seal" offering the path of least resistance for the escaping radiated electromagnetic energy around the inside peripheral walls of the access opening 18. In addition to the primary seal, additional safety measures may be provided by a "secondary seal" along the frontal edge of the oven enclosure and casing walls which will absorb any energy bypassing the choke arrangement. A first energy absorbent member 64 is affixed around the peripheral frontal edge of the walls defining the access opening 18 and a similar absorbent material 66 is disposed along the peripheral walls of the door ring member 30 with the absorbent members 64 and 66 in contiguous relationship upon the closing of the door assembly 20. For the purposes of the secondary seal, a dielectric plastic material such as vinyl is loaded with a highly energy-absorbent material such as carbon black. The seal materials may be secured by an adhesive material to the metallic walls.

Referring next to FIG. 5, the electrical circuit parameters of the choke arrangement will now be described. To assist in this explanation, numerals designating structure similar to that shown in FIG. 4 has been similarly numbered. The first electromagnetic energy transmission path 56 is defined by walls 14 and 28a. A second electromagnetic energy transmission path 60 substantially parallel to the first path is defined by conductive walls 28a, 28, 30 and 30a. A terminating conductive surface wall is thereby provided by the door panel member 28. The dimension from the point designated by the letter A to the midpoint of the opening between walls 28a and 30a designated by the letter B is approximately one-quarter wavelength of the operating frequency. Similarly, a distance of one-quarter wavelength is provided between the point B and the terminating conductive wall surface indicated by the letter C. The total overall choke transmission path thereby provided is approximately equal to an integral number of one-half wavelengths from the point A. As a result of these parameters, an electrical short circuit or low impedance at the point C is reflected as a short circuit at the point A or point of origin 58 of the first transmission path 56. As a result, electromagnetic energy escaping from the oven will be prevented from leaking from the edges of the door indicated by point D.

It will be noted that in the fabrication of the overall door assembly, the door ring member 30 is secured to the panel member 28, illustratively, a plurality of spot welds 68 spaced a predetermined distance apart. The leakage electromagnetic energy traversing the one-half wavelength choke will cause high surface currents in the vicinity of the inner portion of the wall surface 28 at the point C as well as walls 28a and 30. The surface currents within the second transmission path 60 are indicated by the arrows 70 and 72. Such high surface currents in the vicinity of the terminating conductive surface and second transmission path must be considered in the establishment of the distance between the spot welds as well as the selection of conductivity characteristics of the metallic materials for the door ring and panel members. Any fault in fabrication will readily defeat the purposes of the choke arrangement to thereby result in undesirable electromagnetic energy radiation. In addition, the high electrical resistances arising will eventually result in a high impedance at the point A rather than an electrical short circuit and cause an extremely high leakage of energy around the point D at the edges of the door structure.

To circumvent the undesired safety hazards as well as permitting the selection of lower cost materials as well as lower electrical conductivity characteristics, the invention provides for the disposition of a relatively thin layer of conductive material forming substantially a double wall with at least one of the underlying perimeter wall surfaces supporting the high electric surface currents within the second transmission path 60 within the choke arrangement. The high surface currents will thereby be supported by the conductive layer material. The second transmission path of the choke arrangement then, is analogous to a shorted transmission line with essentially a high voltage node and low surface currents at the point B which will be the open end of the conductive material. In view of the absence of high surface currents at this point, the addition of the layer material will have no adverse effect on the overall operation of the microwave oven door primary seal.

In FIGS. 6 to 8 inclusive, several embodiments of the invention incorporating the conductively lined choke arrangement are illustrated. There are numerous suggested methods for the application of the layer material of any suitable conductive metal having a thickness of approximately 0.004--0.008 inches or any thickness satisfactory for supporting the high surface currents along the preferred wall surface areas. A metallic foil such as aluminum, gold or silver may be glued on one side for application of the layer material to the metallic wall surfaces. In FIG. 6, the layer 74, it will be noted, is applied to the surfaces 28a, 28, 30 and 30a. The dielectric filler member 62 may then be disposed within the choke in contiguous relation with the conductive foil layer 74. Alternatively, the invention may be practiced by coating or plating the outer surfaces of the dielectric body 62 with a plating thickness of only a few thousandths of an inch over the preferred wall portions. It will be noted that numerous ways for the application of the conductive layer will occur to those skilled in the art and in configurations of the choke structures disposed in an alternative manner similar to those disclosed in the aforementioned United States patent. The dielectric body member may be also omitted and the free space employed.

Since the conductive layer supports the high electric surface currents, the use of the electrical welding techniques may be omitted and mechanical securing means such as rivets 76 may be utilized to secure the door ring member 30 to the door panel 28. The electrical resistance of the door components, therefore, will no longer have to be considered with relation to the desired choke electrical parameters to support the electromagnetic leakage energy.

FIG. 7 illustrates an alternative embodiment of the invention with the conductive foil layer material 78 disposed on walls 28a, 28 and 30 defining the second transmission path. The conductive foil layer 78 has a substantially U-shaped configuration which may facilitate the assembly and support of this component by the dielectric body member 62.

In FIG. 8, still another alternative embodiment of the invention is disclosed with the choke arrangement 80 provided as a part of the oven wall 14 instead of the door assembly. The conductive choke layer 82 is disposed similarly to the orientation in FIG. 6 with the exception that oven walls 14 are now contacted. Mechanical securing means 86 will secure choke ring 14a to oven walls 14. In this embodiment panel member 88 and door ring 90 are made of a single metal sheet. The resultant choke arrangement will thereby be provided around the entire periphery of the oven access opening.

The resultant microwave oven apparatus with the conductive layer, along the choke walls supporting the higher electrical surface currents has been found to provide an effective seal under all operating conditions and maintained the radiation leakage well below 10 mw./cm.2 the accepted safety standard established for such apparatus. Additional energy seal arrangements with numerous modifications, variations or alterations may be practiced by those skilled in the art and are considered to be within the spirit and scope of the invention as defined in the appended claims. It is intended, therefore, that the foregoing description be considered as illustrative only and not in a limiting sense.