CA1114455A - Radiating mode stirrer heating system - Google Patents

Abstract

Abstract of the Disclosure A microwave heating system having a rotary multiport microwave radiator positioned in a cavity and having radiating ports at different distances from the axis of rotation which direct overlapping radiation patterns at a body to be heated.

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Description

Background o~ ~he Illverltion ~ licrowave ovens have used cavities containing mode stir-ring structures to provide varying clectric field pa~terns in the cooking area of the oven by in~roducing microwave energy -into a cavity, wllich generally has interior dimensions large with respect to a wavelength of the microwave frequency, and moving conductive elements in the cavi~y to reflect the energy and vary the patterns so that points of maximum voltage gra-dients are continuously shifted in the cavity to more uniformly heat different sizes and shapes of bodies. Such a reflective mode stirrer, which is designed for one set of load conditions such as heating hamburgers or hot dogs, does not produce the same effectiveness in- uniformly heating a large body such as a roast or a wide relatively planar body such as a pie.
Coaxial feeds of microwave energy into microwave heating cavities with mode stirrers rotating concentric with the feed have still provided that the mode stirring be by reflection from metal members moving with respect to the food body as shown, for example, in U.S. patent No. 3,436,507, issued on April 1, 1969 to H. A. Puschner.
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Summary of the Lnvcntion ~ he present invention provi~es for a mode stirrer in a microwave oven with microwave energy radiated ~irectly into the oven cavity from a ylurality of movable antenn.l regions in the mode stirring structure so that the electric field mode patterns are varied predominantly by varying the position of the radia-tion patterns radiating from the antenna regions.
This invention further provides that the radiating antennae are ports in the mode stirring structure which are positioned within the cavity such that the radiation pattern of each of the ports covers a region wherein the body to be heated is placed, and such ports are moved with respect to said region so that a substantial portion o~ radiation is absorbed by the body to be heated without reflection from the cavity walls. More speci-fically, this invention provides that such radiation patterns are formed by ports rotating about a common axis at different distances from said axis, hence providing different toroidal re-gions of impingement on the body being heated. IJI addition, the radiating ports preferably are positioned along radii from the axis of rotation which are separated by substantially equal an-gles so that coupling and/or interference between the radiating beam patterns prior to impingement on reflecting walls of the oven is minimized. In addition, the radiating ports are prefera-bly oriented to produce substantial radiation parallel to said .

ax l s, This invention further provides that such a radiation mode pattern provides a region outside the primary radiation pattern prior to reflection of the ovcn walls in which additional heating elements such as resis~ance heaters or flame burner structures may be positioned. More specifically, a resistance hea~ing unit ' ' '~" ~ ' .

~ - 2 -4~i5 may be Eormed with a substantially arcuate shaped portion and be positioned below a mode stirring structurc in the oven cavity and ilaving a radius of curvature larger than thc maximum distance from the axis of rotation of the mode stirring structure to trans-fer heat by radiation and/or convection through the air to a body to be heated without interfering with tlle primary radiation pat-terns of ~he mode stirring structure.
This invention further provides tha~ air may be circulated within the oven by a blower or fan action of the mode stirring structure to assist in the transfer of heat by conduction through the air from the resisti.ve heater and/or to assist in maintaining the oven substantially free of surface wall deposits from con-densed gases driven off from the body being heated.
This invention provides for a coaxial ~eed structure which rotates with respect to a body to be heated about an axis sub-stantially concentric with the coaxial :Eeed while radiating mi-crowa~e energy in a direction predominantlv parallel to said axis whereby annular rings of substant~ially equal energy radiation are produced concentric with said coaxial feed.
This invention further provides for supplying energy through said feed to a heating~cavity having a reflective wall substan-tially perpendicular to said axis o rotation spaced from said radiating mode stirrer, with the ports of said stirrer being positioned at different distances from said axis whereby energy ~: reflected from said wall substantially cancels upon re-enteringsaid feed structure .and, hence 3 is prevented from feeding back to the energy source. More specifically, this invention provides for positioning said ports on radii from said axis which are : spaced apart by 120 degrees to form a three-phase radlating sys-tem, and the magnitude of the power radiated from each port is `
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chosen by choosing the dimensions of the radiating ports~ with said radiating ports being at dlfferent distances along said radii whereby energy reflected from said surface returns to the coaxial radiator at different phases and amplitudes which substantially cancel.
In accordance with the invention there is provided in combination:
a source of microwave energy; a rotating structure fed by microwave energy from said source through means comprising a conduc~ive member supporting said rotating structure; said rotating structure comprising a plurality of micro-wave energy primary radiators positioned at different distances from the axis of rotation of said structure simultaneously supplied with microwave energy from said source for simultaneously radiating at least three separate primary radiation patterns having different polarizations; and means for supporting a body to be heated with said microwave energy in said radiation patterns.
In accordance with another aspect of the invention there is provided a radiating structure for a microwave oven comprising: a plurality of primary radiators simultaneously radiating patterns having different electric field polarizations and fed from a common microwave source by means comprising a conductor extending through an aperture in a wall of said oven; said radiators ...
being spaced from a reflecting surface by substantially equal distances; and 2Q each of said radiators being positioned a different dis~ance from a common point on the transmission line system coupling said source to said~radiators.

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Brief ~)cscription of the Drawings Other and further objects and advantages of the invention will become apparent as the description thereof progresses, reference being had to the accompanying drawings wherein:
Fig. 1 illustrates a vertical sectional view taken along line 1-1 of Fig. 2 of a microwave oven embodying the invention;
Fig. 2 illustrates a horizontal sectional view of the oven illustrated in Fig. 1 taken along line 2-2 of ~'ig. 1;
Fig. 3 illustrates an enlarged view of the mode stirrer section of Fig. l;
Fig. 4 illustrates an enlarged detail of the mode stirrer section of Fig. 2; and Fig. 5 illustrates a further embodiment of the invention.

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~ ~ : .: 5 ~ L~ 3 Description o~ the Preterrecl rmbocLiment Referring now to ~igs. l and 2, there is shown a microwave oven 10 comprising a cooking enclosure 12 formed of metal and having a door 14 which closes an access opening in the enclosurc 12.
Positioned in oven 10 is a micro~ave energy :feed structure 16 comprising a flat plate structure 18 having slots 20 therein through which microwave energy radiates into the interior of the oven. A microwave feed cavity formed by a plate 22 positioned below plate 18 and connected to plate 18 by an outer wall mem-ber 24 is supplied with microwave energy by a coaxial line 26 .
whose outer conductor 28 is fixed with respect to the oven and whose inner ~onductor 30 extends outside the oven to a motor 32 which rotates feed structure 16 about an axi.s concentric with coaxial feed 26.
Outer conductor 28 of coaxial line 26 is connected to wave-guide 34 while inner conductor 30 extends through the waveguide 34 to feed microwave energy from the waveguide 34 through the feed structure 16 into ~he enclo~ure 12. Waveguide 34 is sup-: 20 plied with microwave energy from a n-agnetron 36 in accordance ~with well-~nown practice.
~ As shown ln detail in Figs. 3 ~ 4 impedance matching struc- ~ .
:~ ~ tures 38 and 40 around coaxial feed 30 provide transitional im-:~ : pedance matching bet~-een the waveguide 34 and the coaxial line .. .: -~ 26 and between the coaxial line Z6 and the microwave feed struc-: ~ ture 30. Structure 38 also acts with conductor 30 as a choke to ; prevent microwave energy from leaking out toward motor 32.
: : The openings 20 are radiating antennae, also ref0rred to as radiating ports, positioned at different distances from the ~
axis of rotation of the structure 16 and are shown, for example, :: ~

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as three openin~s oriellted a~out said axis at 12~-de~rce angles witll respect to each otll~r. Ille polts ~0 are spaccd from the axis of rotation 16, ~or ~xample, by different dis-tances.
While distances of ports 20 from said axls may be other than those shown, such distances prefera~ly differ by amounts which cause energy reflected from the opposite oven wall to cancel in the stirrer feed cavity due to out of phase summa-tion at the center conductor 30. In addi-tion, the apertures 20 are preferably spaced at right angles to radii of the axis of rotation as elongated slots perpendicular to said radii, with the length of said slots being on the order of a wavelength of the energy and the width of the slots being less than a quarter wavelength of the energy so that the radiation from the slots will be in the TEM mode with the el-ectric lines parallel to the radii through the a~is of rota-tion of the structure.
The power radiated from each port 20 is dependent on the width of that port, and any desired pattern can be achieved by selecting the port width. However, the radiating port furthest from the axis of rotation preferably radiates the most power. Also, the distance from the axis of rotation to -~
the center of the innermost slot at the end of the slot is preferably nearly as great as the distance radially from the ::
axis of rotation to the center of the closest portion of the next slot and, similarly, the distance of the end portions of the middle slot are less than the distance from the axis of ~: :
rotation to the closest point of said furthest slot so that when the structure 16 rotates, the areas of the slots sweep out ov~erlapping toroidal regions.

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A wi.re grill 46 having openin~ ~limensions gr~ater than a wavc-length of said r~diation is l)osi-tioncd abo-ve th~ ra~iating struc-ture 16. The posi.tion of grill 46 i~ adjustable by the structure 46 being slid in and out between bun-ps 48 and the side walls of the enclosure 12.
Positioned on support rack 46 is a dielectric plate 50 of, for example, pyroceram having a dielectric constant at the micro-wave frequency which is greater than unity and may be, for exam-ple, on the order of ten depending on the particular ceramic. Due to the difference in dielectric constant, the field pattern radia-ted from the slots 20 is caused to converge slightly into a food body 52 supported thereon. Food body 52, as shown, is supported in a dish 54 and may constitute, for example, a roast of meat or other food bodies to be heated or cooked.
Positioned around the outside of rotating feed 16 are resis-tive heating elements 56 and 58 which may be used before, after, or during the application of microwave energy to the food body 52.
Elements 56 and 58 may, for example, in a typical oven have a re-sistive heating capacity of one kilowatt per element and heat ~he ~ :

2~ oven and the body by radiation as well as by convection.
, The motor 32 and feed structure 34 are positioned outside the enclosure 12 whose outside is preferably insulated, for example, by insulation 60 held in place by an outer oven skin 62. Therefore the waveguide structure 34 and coaxial oven feed are not overheated when the resistance heating elements are ~
operating. In addition, cooling air is supplied by a blower ~:
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; 64 driven by an electric motor 66 which cools the anode of :.
; magnetron 36 by blowing air past fins on the magnetron and .
cools waveguide structure 34 by blowing air into l~aveguide -~
structure 34 through apertures 68 in the waveguide structure '~ "

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3~. A portion of t~lC air is l)lown tilrough the coaxial feed 26 and out into the oven through l~orts 20 to aid in circuia-tion of the hcat in the oven and to e~haust cooking gascs through apertures ~0 in the enclos~lre 12, suc!l vapors being processed in a canister 72 in accordance ~ith well-known practice so that the air exhaustcd from canister 72 may be exhausted directly into the kitchen.
Referring now to Fig. 5, there is shown a modification of the invention wherein two rotary feed mode stirring struc-tures are positioned in a microwave heating cavity 80. More specifically, microwave heating cavity 80 has a mode stirring structure 82 positioned in the bottom thereof fed through the floor of the cavity by a coaxial line 84 and rotated by a central conductor 86 of the coaxial line driven by a motor B8 through a belt 90. An upper mode stirring structure 92 is simllarly fed with microwave energy through a coaxial line 94 and rotated by central conductor 96 of coaxial line 94 which is driven by a motor 98 through a belt 100.
Mode stirring radiating structures 82 and 92 which are similar to structure 16 are rotated about a common axis in op-posite directions so that the field patterns radiating from ~the ports 20 in th0 faces 18 of the mode stirrers cross each other as the mode stirrers ro~ate thereby creating additional pattern variation. The port sizes and structures for the mode stirrer may be, for e~ample7 like those disclosed for Figs. 1, 2, 3 and 4. However, other sizes and shapes may be used.
Bodies to be heated 102 are preferably supported on a shelf 104 which is transparent to the radiated energy from stirrer 82 so that the bodies 102 are positioned substantially equîdistant ~ -between the radiating mode stirrers 82 and 92, the bodies 102 .

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being inscrted or remove~l ~ronl the cavi~y ,~() througl) a (loor 10 having a microwave seal l08 ~etween the l~eriphery of tlle door and thc adjacent wall Microwave energy radiclte.l into cavity ~0 may be in the frequency range having a free space wc~veleng~h from one to 100 centimeters. For the batch processor shown herein as, 915 megahertz is preferable while ~or smaller bodies 52, 2450 mega-hert~ may be preferable. The microwave energy is supplied to upper and lower coaxial lines 94 and 84 through waveguide sec-tions 110 and 112, respectively, which are fed from a common microwave source 114, such as a magnetron, through a waveguide 116 and a T-section 118. In such a structure, microwave energy radiated, for example, from the upper mode stirring radiator 92 has a portion which passes through food bodies 102 without absorption to impinge on lower radiator 82 and a portion there-of is coupled back through coaxial line 84 to the waveguide 110. However, since such energy on reaching the T section 11 will have only a fraction thereof coupled back to the magnetron 114 with the rest being coupled to the waveguide portion 110 and back to the mode stirring radiator 92, the isolation of the-magnetron 114 from energy fed back from the cavi~y is ~reater than that which would OCCUI' i f only one of the mode s$irring radiators ~2 and 92 were used. For this reason~ the magnetron 114 may have its output coupled closer to the wave-guide 116 and hence closer to its maximum efficiency operating conditions without changes in the energy absorption produced by different points of the heating cycle of the load 102 or different loads, causing excess reflection of power to the mag-netron l14 which could damage the magnetron by overheating ~: : :

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While the magnetron ll~ is shown herein as cooled ~y air from a blower 120, a ~ater cooled magnetron could be uscd and, in any event, some oE the air ~rom the blower 10 is preferably cc pled into the waveguide 116 through ports 122 to be directed through the waveguides 110 and 112 and the coaxial lines 84 and 94 into the~cavity 80 to carry away gases produced by the heating which are exhausted th-rough an outlet canister 124.
This completes the description of the embodiments of the invention illustrated herein. However, many modifications thereof will be apparent to persons skilled in the art without departing from the spirit and scope of this invention. For example, the mode stirring radiators could be moved in paths other than circular, the feeds to the mode stirring structures could be other than coaxial lines such as, for example, wave-guides, and the structure could be used in continuous processing applications in which a conveyor belt moves bodies to be heated past the rotary mode stirring radiators. Accordingly, it is desired that this invention be not limited by the parti-cular details illustrated herein except as defined by the appended claims.

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Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In combination:
a source of microwave energy;
a rotating structure fed by microwave energy from said source through means comprising a conductive member supporting said rotating struc-ture;
said rotating structure comprising a plurality of microwave energy primary radiators positioned at different distances from the axis of rotation of said structure simultaneously supplied with microwave energy from said source for simultaneously radiating at least three separate primary radiation patterns having different polarizations; and means for supporting a body to be heated with said microwave energy in said radiation patterns.

2. The combination in accordance with Claim 1 wherein:
said radiators are fed by said source of microwave energy through means comprising a coaxial transmission line supplying microwave energy to all of said radiators.

3. The combination in accordance with Claim 1 wherein:
said microwave energy is supplied to said rotary structure through means comprising a waveguide.

4. The combination in accordance with Claim 3 wherein:
said rotary structure is coupled to said source through a coaxial transmission line coupled to said waveguide.

5. The combination in accordance with Claim 1 wherein:

said body to be heated is supported in an enclosure surrounding said body and having an access opening closed by a closure member; and the central conductor of said coaxial transmission line extends through an aperture in a wall of said enclosure.

6. A radiating structure for a microwave oven comprising:
a plurality of primary radiators simultaneously radiating patterns having different electric field polarizations and fed from a common microwave source by means comprising a conductor extending through an aperture in a wall of said oven;
said radiators being spaced from a reflecting surface by substantial-ly equal distances; and each of said radiators being positioned a different distance from a common point on the transmission line system coupling said source to said radiators.