Combination microwave oven control system

A self-cleaning combination microwave and electrically heated oven having a rotating microwave radiator positioned beneath a microwave transparent protective cover of high temperature material with air being blown by a microwave source into the oven through the radiating system and impinging on the cover, thereby cooling and assisting in preventing deposition of cooking vapors on the cover. During the self-cleaning cycle, air is drawn by convection through the oven to maintain the radiating system below 650.degree. F. while the interior oven surfaces are raised to temperatures above 900.degree. F. by resistive-heating elements in the oven.

Application Ser. No. 855,051, filed Nov. 25, 1977, by Bernard J. Weiss and 
assigned to the same Assignee as this application, is hereby incorporated 
by reference and made a part of this disclosure. 
BACKGROUND OF THE INVENTION 
In the aforementioned application, there is disclosed a combination 
microwave and electrically heated oven in which microwave energy is 
supplied at the bottom of the oven. However, in such an oven, the 
radiating applicator is beneath the food body being heated so that 
spillage or other undesirable effects can occur which reduce the 
efficiency of the cooling system. In addition, if high temperature 
self-cleaning is used, the radiator must be made of material, such as 
steel, which will withstand such temperatures. 
SUMMARY OF THE INVENTION 
This invention discloses that a desirable heating pattern can be achieved 
with a multi-element radiator fed from a common junction and producing a 
plurality of substantially directional patterns of radiation toward a body 
to be heated with a smaller spacing between the radiator elements than 
previous radiators by causing at least one of said elements to radiate a 
pattern whose polarization lies in a plane having a substantially 
different angle to the plane of radiation polarization plane, the radiator 
pattern produced by a different element of said radiator. More 
specifically, the polarization of one of said radiation patterns lies in 
the plane substantially parallel to the axis of rotation of the radiator. 
The radiator elements are preferably ports in a waveguide plenum fed by a 
common junction such as a coaxial line with the microwave energy arriving 
at different radiating elements in respectively different phases. The 
improved uniformity of the heating pattern in a food body produced by such 
independent radiation patterns occurs even when the food body is 
positioned directly in the direct radiation patterns of the radiating 
elements so that a substantial portion of the energy is absorbed by the 
food body as direct radiation from the radiator rather than reflected from 
the cavity walls. 
This invention discloses that a stationary cover which is transparent to 
microwave energy may be positioned over the radiator in the oven cavity to 
provide protection of the radiator from spillage of food or condensation 
of cooking vapors on the radiator. More specifically, the transparent 
cover is supported on bumps on the bottom wall of the oven cavity 
providing spaces through which air passes through the oven. The air 
directed out through the radiator ports past inner surface portions of the 
cover adjacent the radiator to maintain the radiator at a temperature 
below that of the oven when electrical resistance heating is used. More 
specifically, the cover may be, for example, a commercially available high 
temperature material such as a pie plate sold under the trade names 
Pyroceram or Rayceram which will withstand temperatures in excess of 
1000.degree. F. The pie plate cover is inverted in a stationary position 
over the rotating radiator and acts as a thermal shield during high 
temperature oven operations such as a cleaning cycle to allow the radiator 
to remain at temperatures substantially below temperatures of the oven 
wall surfaces. 
In accordance with this invention, the walls of the oven are preferably 
made of conventional steel coatel with high temperature ceramic such as 
enamel so that in the absence of large load bodies, microwave energy being 
radiated into the oven will be partially absorbed by the oven walls, hence 
preventing the build-up of undesirable high intensity field patterns in 
the oven. The radiator may be of a high conductivity metal such as 
aluminum which is protected from the high temperature of the oven. 
This invention further discloses a microwave oven having a door seal 
structure incorporated with a conventional high-temperature gas vapor seal 
for a combination oven and a low microwave loss microwave choke seal 
structure between the vapor seal and the oven interior which allows air to 
be blown continuously through the oven without substantially reducing the 
heating rate of the food body by microwave energy or by resistive heating. 
More specifically, the choke structure may be formed either in the door or 
in the oven wall and provides a high impedance in series with the input 
transmission line structure at the predominant operating frequency range 
of the microwave oven, such as, for example, between 2.4 to 2.5 KMH. 
In accordance with this invention, additional resiliency may be provided 
for the resilient high-temperature vapor seal by supporting wire mesh in 
tubular form on a tubular fiberglass braid in turn supported on a tubular 
steel mesh structure of greater diameter wire to provide spring action. 
In accordance with this invention, sealing action of the microwave energy 
in the predominant frequency range may be enhanced by providing means in 
an input transmission line section of a choke-type microwave door seal for 
inhibiting transmission of such microwave energy periphery around the said 
input transmission line structure. More specifically, such means may 
comprise impedance discontinuities such as slots in one of the walls of 
said input transmission line structure. Further in accordance with this 
invention, such slots preferably extend through the wall into the choke 
structure to further assist in inhibiting periphery mode propagation in 
the choke structure. Such a choke seal is positioned inside the oven 
between the high-temperature vapor seal and the oven interior. 
In accordance with this invention, a food body may be positioned on a rack 
in the radiation patterns from a rotating radiator, preferably formed of 
high conductivity, low loss material such as aluminum, so that a 
substantial portion of the microwave energy is absorbed on passing through 
the food body prior to reflection from walls of the oven. Therefore, high 
efficiency heating may be achieved with microwave energy even though the 
walls of the oven are made of inexpensive high-temperature material such 
as enameled steel, which slightly absorbs microwave energy. In accordance 
with this invention, the magnetron may be tightly coupled to the oven 
through a coupling mechanism such as a waveguide, coaxial transition and 
multi-element radiator thereby increasing the efficiency of the magnetron 
and hence conversion of input electrical energy to microwave energy 
coupled into the body to be heated. More specifically, where a small food 
body on a metal dish is placed in the oven, substantial microwave energy 
which is radiated into the oven, is reflected back to the rotating 
radiator from the metal dish or the opposite wall of the oven will arrive 
at a common junction, such as the central conductor of a coaxial line 
transition, with substantially different phases so that relatively low 
amounts of energy are reflected back into the magnetron. 
In addition, an electric resistance heater, which may be positioned 
directly in the top of the oven while extending through microwave chokes 
in the oven wall for connection to an electrical power source, is used for 
heating the oven, either separately or in combination with microwave 
energy, and is used for treating the oven walls during the self-cleaning 
cycle. 
When microwave energy alone is being used, deposition of cooking vapors on 
the surfaces through which the microwave energy passes is inhibited by 
re-heating of such surfaces by the microwave energy. However, this 
somewhat reduces the efficiency of the transfer of microwave energy to the 
body to be heated, and accordingly, air directed past the surface of the 
cover is first heated by passing such air by the generator, such as a 
magnetron, to cool the magnetron and to heat the air to a temperature on 
the order of 40.degree. F. above room temperature, and hence above 
100.degree. F., so that substantially no cooking vapors will deposit on 
the transparent cover. In accordance with this invention, the air heated 
by the magnetron flows through the oven without substantially cooling the 
oven so that clouds of steam or other cooking vapors are carried away 
during the cooking process and are not emitted from the oven through the 
door when it is opened.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIGS. 1, 2, and 3, there is shown a microwave cavity 10 
closed by a door 12 and supplied with microwave energy from a rotating 
radiator 14 in the bottom of the oven. Radiator 14 is fed with microwave 
energy from a magnetron 16 through a waveguide 18 and a coaxial line 20 
having a central conductor 22 rigidly connected to rotating radiator 14 
and extending through waveguide 18 to a gear reduction motor 24. Motor 24 
is attached to the bottom of waveguide 18 and rotates central conductor 22 
to rotate radiator 14. Coaxial line 20 has an outer conductor 26 rigidly 
connected to the upper wall of waveguide 18 and extending through the 
bottom wall of enclosure 10 into a plenum 28 in radiator 14. 
As shown more specifically in FIG. 5, plenum 28 comprises an upper plate 30 
connected to central conductor 22 and having a plurality of ports 32 
therein spaced at different distances at the axis of conductor 22. 
Microwave energy is radiated from plenum 28 into the oven enclosure 10 
through ports 32 and through a stationary cover 34 which is transparent to 
microwave energy and which prevents cooking spillage or vapors from 
impinging on radiator 14. 
A lower plenum member 38 of radiator 14 prevents radiation of microwave 
energy radially outwardly and directs it through the ports 32, and the 
lower surface of plenum member 38 is positioned sufficiently above the 
bottom wall of enclosure 10 for radiator 14 to rotate freely. An aperture 
in plenum member 38 surrounds the upper end of outer coaxial conductor 26 
which thus extends slightly into plenum 28, thereby substantially 
preventing microwave energy from radiating into enclosure 10 from beneath 
radiator 14. The length of outer conductor 26 which extends into plenum 28 
may be adjusted to improve impedance matching conditions. 
As shown in FIG. 1, a substantially conical waveguide to coaxial line 
transition member 40 is formed by die-stamping the sheet metal bottom wall 
of guide 18 upwardly in conical shape surrounding central conductor 22. A 
tubular member 42 is welded to the top of conical member 40 and extends 
downwardly surrounding conductor 22 for distances equal to an effective 
electrical quarter wavelength at the frequency of magnetron 16 so that it 
produces a choking action to energy attempting to escape from waveguide 18 
toward motor 24. A sleeve bearing 44 of dielectric material is positioned 
between tubular member 42 and conductor 22 to insure against arcing in the 
bearing. Conductor 22 is reduced in diameter just below the lower end of 
tubular member 42 producing a land resting on an oil-filled bronze bearing 
46 supported in a plate 48 which attaches motor 24 to the bottom of guide 
18. The ends of waveguide 18 are closed by shorting members which are 
positioned to provide a substantially flat standing wave ratio between the 
output probe of magnetron 16 and central conductor 22. 
As shown in FIGS. 4 and 5, radiator ports 32 are each fed with microwave 
energy through separate plenum waveguide sections whose axes are at 
120.degree. to each other and whose inner ends form a common junction 
region containing the central conductor 22. An impedance matching conical 
portion 52 formed in plenum plate 30 is welded to conductor 22 to increase 
its radius as it approaches upper plate 30 of plenum 28. Waveguide section 
walls which are formed by the sides of plenum 28 are made of different 
lengths so that energy radiated into plenum 28 from central conductor 22 
arrives at ports 32 in respectively different phases. Reference may be had 
to the aforementioned copending application for an additional description 
of the details of such a radiator. 
In accordance with this invention, one of the ports 32 has a vertical 
aperture 54 forming a radiation polarization pattern which is parallel to 
the axis of rotation of radiator 14. Thus, the composite of the heating 
pattern produced by the radiation patterns from ports 32 is different at 
different distances above radiator 14, and the waves from aperture 54 are 
vertically polarized so the waves will arrive at the food body in 
different phases and polarizations, preferably selected so that 
reflections of the waves back to radiator 14 will have substantially 
cancellation of the electrical field vectors at the junction of the plenum 
waveguides at central conductor 22 to re-reflect such energy back through 
ports 32 into the cavity 10. This effect is preferably chosen to be 
maximized when the microwave cavity has no food body positioned therein or 
when the food body is positioned in a metal dish. Under these conditions 
it therefore is possible to couple magnetron 16 to the oven to operate 
close to its maximum efficiency for converting its electrical energy input 
to microwave energy output while maintaining low microwave energy field 
gradients and hence low wall losses in the waveguide 18. 
In accordance with this invention, oven cavity 10 may be made of relatively 
lossy or energy absorbing material which may absorb, for example, a few 
percent of microwave energy impinging thereon and reflecting therefrom. 
Such material may be, for example, conventional sheet steel used in 
conventional ovens and coated with conventional enamel, all in accordance 
with the well-known practice. In addition, conventional broiler and 
heating units 60 and 62 may be positioned adjacent the upper and lower 
walls of the cavity 10 held by conventional fasteners in accordance with 
well-known practice. However, in the case of the heating unit 62, it 
preferably is formed in arcuate shape so that its closest portion is 
positioned around, and spaced from, the periphery of cover 34 so as not to 
overheat the cover 34. 
In accordance with this invention, elements 60 and 62 are shielded electric 
resistance heating units connected to power through the back wall of 
cavity 10. The outer shields of the units are grounded to the wall of 
cavity 10 by tabs 64 which are attached, for example, by welding or 
crimping to the shields and which are screwed to the back wall of cavity 
10. Tubular microwave choke elements 66, whose lengths are preferably an 
effective quarter wavelength of the microwave frequency in cavity 10, are 
attached by welding to the outside of oven cavity 10 and surrounding the 
shields but spaced therefrom by an enamel coating on elements 66. 
Electrical connections to power and control terminals may be made to the 
heater and broiler units in accordance with well-known practice. 
Any desired configuration can be used for the radiator 14. An example 
providing good results at 2.45 KMH uses waveguides which are 4 inches wide 
and 1 inch high fed by a central conductor 22 which is 1/2 inch in 
diameter and an outer conductor 26 which is 2 inches in diameter. The 
waveguide 18, which may be 4 inches wide, is shown as 2 inches high, and 
the distances from one shorting member 50 to the center of magnetron 
output to the axis of conductor 22 and to the other shorting plate 50 are 
3/4 inch, 5 inches, and 101/4 inches, respectively. 
A food body 70 may be positioned, for example, on a rack 72 above radiator 
14 in a dish 74, preferably transparent to microwave energy, and resting 
on a plate 76 of material which is transparent to microwave energy, such 
as pyroceram. Rack 72 may be, for example, a welded wire rod having 
apertures substantially greater than .lambda./2 and adjustably supported 
at different levels in cavity 10 by means of grooves 78 in the side walls 
of cavity 10 or in any other desired manner. 
Air from a blower 80 is blown through the cooling fins of magnetron 16 and 
then into oven 10, for example, through apertures in waveguide 18 via duct 
82, transmission line 20, apertures 32 and spaces between the bottom lip 
of cover 34 and the bottom of oven 10 where the lip of cover 34 rests on 
raised positioning bumps 68 formed in said oven bottom wall, past heating 
unit 62 to conduct that air past food body 70 during cooking. The air then 
exists through a canister 84 at the top of the oven to the center of a 
surface burner unit 86. 
During the oven's self-cleaning cycle, the temperature of the oven is 
raised to a temperature between 750.degree. F. and 1100.degree. F. by 
energizing upper heating unit 60 to vaporize deposits on the wall of oven 
10 and to allow the vapor to move by convection out through canister 84 
which may contain a catalyst to complete oxidation of the vapor in 
accordance with well-known practice. 
Air is also drawn by convection into the oven past radiator 14 and the 
inner surface of cover 34, which may be, for example, Pyroceram, thereby 
maintaining radiator 14 below the temperature where aluminum would soften 
while permitting the upper surface of cover 34 to be heated to 
self-cleaning temperatures for Pyroceram surfaces, such as 700.degree. F. 
to 900.degree. F. 
Thermal insulation 88 of, for example, fiberglass is provided around oven 
10 in a well-known manner surrounded by a metal skin 90. A light 92 may 
illuminate oven 10 through an apertured metal plate 94 covered with a 
translucent pyroceram plate 96. 
In accordance with this invention, door 12 has a high temperature vapor 
seal which is prevented from absorbing large amounts of microwave energy 
from the interior of enclosure 10 by a microwave seal positioned between 
said enclosure interior and the vapor seal. The seal is described in 
greater detail in said copending application. 
The dimensions of the apertures 32 and vertical aperture 54 may be of any 
desired size and are shown here, by way of example, scaled to produce an 
improved heating pattern with a reduced size radiator from that disclosed 
in the aforementioned copending application, however with the distance and 
size of the slot 32 covered by the lip 56 is closer to the axis of 
rotation of the radiator 14 than the farthest slot disclosed in said 
copending application. The lip 56 covering this slot 32 causes the average 
of the radiation pattern radiating from the vertical aperture 54 to be 
directed at an angle toward the rotational axis of radiator 14. The shape 
of patterns of radiation from the vertical aperture 54 and the other 
apertures 32 are governed by size and shape of the apertures 32 and 54. 
Since the radiation pattern from aperture 54 is directed inwardly toward 
the axis of rotation, additional heating occurs in the center of the food 
body. Such a heating is different for different positions of the rack 72 
in the support grooves 78. When the rack is in its uppermost position a 
large portion of the microwave energy from aperture 54 is directed across 
the microwave oven beneath the food body and is reflected from the walls. 
However, when the rack is in its lowermost position a larger portion of 
the energy from the vertical aperture 54 impinges directly on the food 
body, particularly if the food body is large, and it is important to 
couple substantial amounts of microwave energy into the center of the food 
body. The other ports 32 radiate waves that are horizontally polarized in 
a radiation pattern whose average direction is upward, substantially 
parallel to the rotational axis. 
In such a radiating structure provides an improved heating pattern in which 
the major portion of the microwave energy may impinge directly on a food 
body before reflection from the oven walls and the ports are further 
de-coupled from each other for direct radiation so that radiation from one 
port does not affect the radiation pattern from the other ports prior to 
impingement of the pattern energy on the walls of the oven. This is 
achieved even though the spacing between the ports is less than in the 
structure of the aforementioned copending application so that the cover 34 
positioned over the radiation may be of a lesser size. 
Air which is blown through the radiator 14 into the oven largely exits from 
the radiator through the ports 32 and impinges on the inner surface of the 
cover 34 so that it flows along this surface and out under the lip of the 
cover 34 between the bumps 68 which support the cover lip in a raised 
position above the floor of the oven 10. 
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 the invention. For example, other sources of auxilliary heat such as 
gas flames or hot air can be used in place of electrical resistance 
heaters and any desired system of timers and/or controls can be used. 
Accordingly, it is intended that this invention be not limited to the 
terms of the specific embodiment herein except as defined hereby by the 
appended claims.