U.S. patent number 3,857,009 [Application Number 05/395,469] was granted by the patent office on 1974-12-24 for microwave browning means.
This patent grant is currently assigned to Raytheon Company. Invention is credited to Kenneth W. Dudley, George H. MacMaster.
United States Patent |
3,857,009 |
MacMaster , et al. |
December 24, 1974 |
**Please see images for:
( Certificate of Correction ) ** |
MICROWAVE BROWNING MEANS
Abstract
Apparatus is disclosed for heating with microwave energy
utilizing nonresonant means for converting and transforming free
space plane waves within the oven enclosure into fringing electric
field patterns to brown or sear the load surfaces similar to
broiling. One embodiment comprises parallel abutting plates with
alternate plates of a high dielectric constant material to provide
a predetermined phase lag and with the intervening low dielectric
constant sections to provide alternating fringing electric fields,
illustratively, in the pi or any other desired mode pattern.
Alternate embodiments include microwave strip transmission lines
comprising conductive strips separated by a dielectric substrate to
couple and convert the free space waves into the desired heating
mode pattern. A load supporting shelf within the microwave oven may
be readily adapted to include the alternating dielectric materials
to provide the desired fringing electric field patterns. Conveyer
belts may also be adapted to incorporate browning surface means by
strips of a high dielectric material on the belt material. Further
embodiments include microwave oven enclosures radiated by horn
means which may be cross-polarized, as well as, the combination of
horn radiators coupled to microwave browning plates.
Inventors: |
MacMaster; George H.
(Lexington, MA), Dudley; Kenneth W. (Sudbury, MA) |
Assignee: |
Raytheon Company (Lexington,
MA)
|
Family
ID: |
23563174 |
Appl.
No.: |
05/395,469 |
Filed: |
September 10, 1973 |
Current U.S.
Class: |
219/728; 426/243;
219/732; 219/745; 219/762 |
Current CPC
Class: |
H05B
6/6494 (20130101); H05B 6/725 (20130101); H05B
6/6482 (20130101); H05B 6/6411 (20130101) |
Current International
Class: |
H05B
6/80 (20060101); H05b 009/06 () |
Field of
Search: |
;219/10.55 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Reynolds; Bruce A.
Attorney, Agent or Firm: Rost; Edgar O. Pannone; Joseph D.
Murphy; Harold A.
Claims
We claim:
1. In combination:
an enclosure;
means for radiating microwave electromagnetic energy within said
enclosure;
a load; and
means for supporting said load within said enclosure comprising
means for converting and transforming free space energy waves into
a fringing electric field pattern having a substantially
180.degree. phase differential in close proximity to the exterior
surfaces of said load;
said converting and transformer means comprising alternate regions
of high and low dielectric loss characteristics
2. The combination according to claim 1 wherein said converting and
transformer means comprise an array of parallel members of a high
dielectric loss material and alternate intervening spaces with the
walls of said dielectric members facing said spaces being coated
with a conductive material.
3. The combination according to claim 1 wherein said converting and
transformer means comprise an array of parallel members of a high
dielectric loss material and alternate intervening spaces with the
walls or dielectric members facing said spaces being tapered in a
direction to provide the larger intervening space regions in closer
proximity to the load.
4. The combination according to claim 1 wherein said converting and
transformer means comprise a dielectric substrate member having
spaced conductive strips on opposing sides with a greater number of
strips on one side and means for interconnecting alternate strips
on said side with the strips on the remaining side.
5. The combination according to claim 1 wherein said load
supporting means comprise a conveyor belt of a low loss dielectric
material having spaced members of a high dielectric loss
material.
6. The combination according to claim 5 wherein said high
dielectric loss members are disposed in a direction transverse to
the direction of travel of said conveyor belt.
7. The combination according to claim 5 wherein said high
dielectric loss members are disposed in a direction parallel to the
direction of travel of said conveyor belt.
8. Microwave heating apparatus comprising:
an enclosure;
means for radiating microwave electromagnetic energy within said
enclosure;
means for supporting a load within said enclosure;
said load supporting means including means for converting and
transforming free space energy waves into a fringing electric field
pattern having a substantially 180.degree. phase differential in
close proximity to the exterior surfaces of said load;
said converting and transformer means comprising regions of
alternately high and low dielectric loss characteristics.
9. The apparatus according to claim 8 wherein said converting and
transformer means comprise a plurality of abutting parallel
dielectric regions forming a plate member.
10. The apparatus according to claim 9 wherein one of said plate
members simultaneously contact opposing surfaces of said load.
11. The apparatus according to claim 8 wherein said load supporting
conveyer belt comprise means of a low loss dielectric material for
transporting a load through said enclosure having spaced strips of
a high dielectric loss material disposed on the load carrying
surface.
12. The apparatus according to claim 8 wherein said converting and
transformer means comprise a plurality of dielectric members of
alternately high and low loss characteristics forming a shelf
within said enclosure.
13. The apparatus according to claim 8 wherein said energy
radiating means are disposed below said load supporting means.
14. The apparatus according to claim 8 wherein said load supporting
means comprise a dielectric substrate member having spaced
conductive lines on opposing sides with the number of lines on one
side being substantially greater and means for electrically
interconnecting said lines to provide the predetermined fringing
electric field pattern.
Description
BACKGROUND OF THE INVENTION 1. Field of the Invention
The invention relates to microwave heating and apparatus for
producing broiled surfaces on a radiated load.
2. Description of the Prior Art
The heating of articles by microwave energy has become widely
accepted both here and abroad in recent years due to the rapid
preparation times resulting from the so-called "dielectric heating
phenomenon." Such apparatus typically includes an energy source
such as a magnetron with the energy fed within a conductive wall
oven enclosure through waveguide transmission means. The
electromagnetic waves are radiated and reflected within the
enclosure in free space and are distributed by such means as mode
stirrers to uniformly surround and be absorbed by the load to
result in high frequency oscillatory movements of the molecules to
cause heating by molecular friction. The allocated frequencies for
such heating apparatus are assigned by the Federal Communication
Commission and are 915.+-. 13 MHz and 2,450.+-. 50 MHz. The term
"microwaves" is intended to define electromagnetic energy radiation
having wavelengths in the order of 1 meter to 1 millimeter and
frequencies in the order of 300 MHz to 300 GHz.
All materials exposed to electromagnetic radiation have differing
dielectric constant and loss tangent characteristics and,
therefore, the rate of heating becomes a varying factor.
Accordingly, the varying absorption of the radiated energy causes
the depth of penetration and surface coloration of loads to vary.
Where a browned surface is desired, similar to broiling, this is
difficult to achieve without long exposure which results in
overcooking of the interior regions because the microwave oven
enclosure is cooler relative to electric and gas apparatus so that
the exterior load surfaces tend to be cooler than the interior and
there is heat loss due to evaporation of moisture. Microwave cooked
exterior surfaces, therefore, have been treated in a slightly
different manner to produce the desirable coloration.
Prior art techniques for browning include the incorporation of
electric or gas broiling elements in the microwave oven. Another
method involves the coating of the outer surfaces with a food
additive having a higher energy absorbing characteristic which will
lead to more rapid heating of the outer surfaces, while the
remainder of the load achieves the desired degree of cooking. Still
another example of prior art teachings involves the use of lossy
ovenware or utensils having a selective heating capability by means
of the use of conductive materials. Such conductive materials may
be incorporated in a shelf of a dielectric material supporting the
load or comprise radiating means having a plurality of rods with a
predetermined spacing to cause the rapid absorption of the
microwave energy by the rods as shown in U.S. Pat. No. 3,591,751,
issued July 6, 1971 to C.E. Goltsos U.S. Pat. No. 2,830,162, issued
Apr. 8, 1958 to D.A. Copson et al. and assigned to the assignee of
the present invention, utilizes a utensil of a ferromagnetic
material responsive to the energy impinging thereon up to the Curie
temperature point. Thereafter, the utensil becomes substantially
nonresponsive and is pervious to the electromagnetic energy.
Ferromagnetic materials sustain the alternating electric and
magnetic fields and, characteristically have high energy loss at
the temperatures below the Curie point. Such materials include
alloys of manganese, tin and copper, or manganese, alumnium and
copper as well as alloys of iron and sulphur, such as pyrrhotite,
whose crystals have the form of hexagonal prisms. Examples of such
materials are zircomates of lead and barium and the titanates of
lead, barium and strontium.
Other examples of the prior art are found in U.S. Pat. No.
3,219,460 issued Nov. 23, 1965 to E. Brown which discloses
containers of a dielectric material having electrically conductive
shields such as aluminum wrapped completely around the side and
bottom surfaces. Other geometric patterns are achieved in the
aluminum foil. U.S. Pat. No. 3,302,632 issued Feb. 7, 1967 to E.C.
Fichtner also relates to a plastic-type package having varying
microwave transparent characteristics. Such containers are readily
adaptable to the preparation of frozen foods, such as T.V.
dinners.
The prior art radiant heaters, selected heating packaging and lossy
utensils have been utilized, however, a need still exists for a
simple, less costly apparatus which will not be subject to
breakage; require additional cleaning or reduce the amount of power
available for heating.
SUMMARY OF THE INVENTION
In accordance with the teachings of the present invention a plate,
dish, shelf or any load supporting means is provided with
substantially parallel alternating regions of varying dielectric
characteristics to result in intense alternating electric fields in
close proximity of a load with said fields rapidly decaying a short
distance from the load surfaces. The pi-mode or electric fields
180.degree. out-of-phase is an illustrative electric field pattern
for browning the load surfaces. The free space electromagnetic
waves within the oven enclosure are converted by the
nonfrequency-responsive and nonresonant means comprising the
dielectric members from plane waves into the desired alternating
fringing electric field patterns. The alternate dielectric members
of high dielectric loss materials provide a phase reversal or phase
lag relative to the intervening low loss dielectric or air-filled
members which provide a different propagation characteristic. There
are no size limitations other than the oven enclosure and there is
no frequency dependency so that the invention is readily adaptable
to conveyer systems or complete shelves in addition to plates or
utensils to provide the fringing field patterns.
Alternate embodiments of the invention may be realized using strip
microwave transmission line techniques providing a capacitor effect
between conductive strips on opposing sides of the dielectric
substrate. A plane wave entering the bottom of the microwave
coupling and transformer means will effectively charge all the
conductive strips to convert the waves into the desired mode
patterns. Alternatively, all the embodiments of the invention
utilizing parallel plates or conductive strips of varying
propagation characteristics may be disposed on both sides of the
load to shorten the browning time. Ceramic materials having a
dielectric loss of 25-50 have been utilized in exemplary
embodiments and a rule of thumb for the high dielectric materials
for the energy coupling and transformer browning means to obtain
the desired phase reversal would be to use a material having a
dielectric value approximately equal to the square root of the
dielectric constant value of the material being heated.
In all embodiments it is noted that in view of the rapidly decaying
fields a short distance from the load relatively little splattering
or heat is generated along the oven walls. Further, the materials
utilized can be readily cleaned and are of a more durable nature
than the lossy type ovenware utilizing ferromagnetic materials
which become very brittle and are easily broken at the temperatures
encountered in the oven. The invention may also be adapted to any
desired mode field patterns such as TM.sub.01 which would be useful
for illustratively baking potatoes. A rotisserie arrangement is
also possible and horn radiators for feeding the microwave energy
into the oven enclosure are utilized with the horns being
cross-polarized. The microwave coupling and transformer means can
also be used in combination to couple the energy from the feed
means to further enhance the advantages of microwave cooking.
BRIEF DESCRIPTION OF THE DRAWINGS
Details of the illustrative embodiments of the invention will be
described with reference to the accompanying drawings, wherein;
FIG. 1 is a vertical cross-sectional view of the microwave
apparatus embodying the invention;
FIG. 2 is a diagramatic representation to assist in the
understanding of the invention;
FIG. 3 is a diagramatic representation of a browning plate
embodiment of the invention illustrating the alternating fringing
electric field orientation along the top surface;
FIG. 4A is a top view of a partial portion of a conveyor belt
embodiment utilizing the invention;
FIG. 4B is a diagramatic representation of the alternating fringing
electric fields disposed in close proximity to the top surface of
the embodiment in FIG. 4A;
FIG. 5 is a top view of a browning plate embodiment shown
supporting a load;
FIG. 6 is an end view of the browning plate embodiment shown in
FIG. 5;
FIG. 7 is a bottom view of a strip microwave transmission line
embodiment of the invention;
FIG. 8 is a cross-sectional view taken along the line 8--8 in FIG.
7;
FIG. 9 is a cross-sectional view of an embodiment of the invention
utilized as a shelf in a microwave oven;
FIG. 10 is a perspective view of a microwave oven adapted for a
rotisserie and radiated by horn-type radiators with cross-polarized
fields; and
FIG. 11 is a perspective view of the apparatus shown in FIG. 10 in
combination with the microwave coupling and transformer browning
means embodying the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A typical microwave oven apparatus 10 found in the art today is
illustrated in FIG. 1. Top and bottom conductive walls 12 define
with sidewalls 14 the resonant oven enclosure 16 having
predetermined dimensions to support a plurality of free space
wavelengths at the apparatus operating frequency. At least one of
the sidewalls 14 is provided with an access opening which is closed
by means of a door and choke assembly commonly used in such
apparatus of the side or bottom-hinged type. A case 18 surrounds
the oven enclosure and is provided with a front panel member 20 for
mounting timers 22 and 24, as well as, buttons 26, 28 and 30 for
actuation, respectively, of the start, stop and light circuits.
A magnetron energy generator 32 of the type described in the text
"Microwave Magnetrons," Radiation Laboratory Series, Volume 6, by
G.B. Collins, McGraw-Hill Book Company, Inc., 1948 is mounted
behind the front panel member. Such sources are energized by
rectified line voltages of approximately 3,000 to 6,000 volts DC
and the electrical circuits, as well as details of the energy
source have been purposely omitted since they are considered to be
well-known in the art. The microwave energy is coupled to the
resonant cavity enclsoure 16 by means of an antenna 34 within a
dielectric member 36 extending into the launching rectangular
waveguide 38. A terminating end wall 40 is disposed at one end of
the launching means spaced approximately one-quarter of a
wavelength from the antenna 34 for maximum launching efficiency.
The opposing end is open as at 42 to provide for the radiation of
the energy within the enclosure. Such launched energy represented
by plane waves 44 is distributed in a cyclically varying manner by
such means as a mode stirrer 46 having a plurality of vanes 48
which are rotated by a motor 50. The energy waves bounce off the
conductive walls at the top, back, side and bottom of the oven
enclosure. The load 52 to be heated and browned, such as a steak or
roast, is supported on the energy coupling and transformer means 54
of the invention which converts the plane waves into the fringing
electric field patterns of rapidly decreasing intensity in close
proximity to the exterior surfaces of the load.
The browning means 54 comprise an arrangement of parallel plate
members having varying dielectric characteristics with members 56
of a high dielectric constant value while the intervening spaces 58
may be filled with a low loss dielectric material or air. The
spaces 58 are lined with a conductive material 60, such as copper
or other metal tape to provide a capacitor effect. Where desired
the energy coupling and transformer browning plate means 54 is
supported on a spacer 62 of a microwave permeable material having a
thickness to provide for the entrance of the plane bouncing waves
from the bottom wall 12. The invention is also practiced by the
provision of another microwave coupling and transformer browning
plate means 64 on the opposing side of the load 52 to thereby
simultaneously brown both sides of the product.
Referring now to FIG. 2 the principle of the invention will be
explained. Plane waves 44 distributed within enclosure 16 have no
specific field orientations and have a wavelength of approximately
4.8 inches for 2,450 MH.sub.z. From the free space region 66 the
plane waves are coupled by mode transformer region 68 to evolve the
fringing electric fields 70 adjacent to the steak 52. It is noted
that the function of transforming of free space waves into the
fringing electric field patterns may be achieved by an integral
assembly or the mode transformer portion may be separate with a
superimposed top plate member. With the invention essentially all
of the energy entering the transformer region 68 is transformed
into a desired heating pattern, illustratively, the pi-mode where
the electric fields alternate 180.degree. out-of-phase as shown
diagramatically in FIG. 3. In this view the assembly comprises a
plurality of parallel plates stacked together in an array with
alternate plates providing a high dielectric constant to obtain a
desired degree of phase reversal or phase lag for the resultant
pattern. To obtain this fringing field configuration plane waves,
indicated by vectors 74, enter the bottom of the browning plate
means 72 and in the low loss or air-filled regions 76 a field
orientation (+) and E-field vector 78 is obtained extending
perpendicular to the direction of transport of the plane waves.
Alternate parallel plates 80 are provided of a high dielectric
material such as K-50 or the material sold under the name Stycast
with the electric field orientations now reversed as shown by
vectors 78. In the reorientation of the plane waves, the H or
magnetic field vector extends in an orthogonal direction,
designated by the vector 82, parallel to the longitudinal axis of
the plate 72. The alternating fringing electric fields are
indicated by arrows 84. These fringing field patterns are of high
intensity adjacent to the top surface of the plate 72 and decay
rapidly in a direction perpendicular to the plate. Unique means for
achieving rapid browning and searing of the exterior surfaces of
load are thereby achieved utilizing the structure of the
invention.
Referring next to FIGS. 4A and 4B another embodiment of the
invention will be discussed. A conveyer belt 90 of a dielectric
material having good microwave transmission characteristics is
provided with a plurality of high dielectric loss material strips
or a coating at alternate intervals, with the strips extending
transversely to the direction of travel of the conveyor, as
indicated by the arrow 94. The invention is not restricted to the
transverse disposition of the alternating dielectric material and
the array, therefore, may be disposed extending parallel to the
direction of travel of the conveyor. FIG. 4B indicates by means of
arrows 96 the disposition of the fringing electric fields in the
region of the exposed conveyer dielectric material while arrows 98
indicate the phase reversal provided by the high dielectric loss
regions 92. It is common in conveyorized systems to have the energy
distributed from a position above or below the conveyor belt as
desired. In either case the invention will provide the fringing
electric fields in close proximity to the conveyor belt
surfaces.
In FIG. 5 a microwave browning plate means 100 is illustrated
carrying a load 102 which is capable of being readily inserted and
removed from the oven enclosure. The parallel plates 104 define the
regions of low loss dielectric material which is readily permeable
to microwave energy and parallel plate members 106 comprise the
high loss dielectric material. In FIG. 6 the configuration of the
high loss dielectric members 106 will be noted having slightly
tapered surfaces 106a and 106b. The waves represented by the
E-vectors 108 entering the bottom portion are coupled and
transformed into the fringing fields exiting from the top surface
as represented by the arrows 110. It will be noted that the
fringing electric fields adjacent to load 102 extend a short
distance from the top surface and, therefore, the desired browning
effect is readily obtained on the exterior surfaces of the load
102.
FIGS. 7 and 8 represent a strip microwave transmission line
embodiment of the invention. A dielectric substrate body 112 is
provided on the bottom side with parallel strips 114 of a highly
conductive materials such as copper. The upper portion of the
substrate 112 is provided with twice as many conductor strips 116.
Alternate conductive strips 116a are connected to the bottom strips
114 by wires 118 to thereby achieve an alternating electric field
as indicated by the + and - signs of the alternate upper strip
members. The unconnected strips form capacitors with the bottom
conductive strips 114. Plane waves fed into the bottom as indicated
by the E-vectors 120 charge all the conductive strips 114 on the
bottom side at the same potential. The unconnected strips 116 on
the top which are separated by spaces 122 result in an out-of-phase
fringing electric field being established across the top surface in
the alternating pi-mode. This configuration of the invention
represents a low cost method of making a nonreasonant means for
coupling and transforming microwave energy into any desired
fringing electrical field pattern.
FIG. 9 illustrates an alternative embodiment of the invention
comprising a shelf member 124 made up of alternate parallel plate
sections of dielectrics 126 and 128. Such an embodiment is mounted
on brackets attached to the oven enclosures sidewalls 14 to space
the shelf at any desired height from bottom walls 12. Parallel
sections 126 comprise the low dielectric loss material while the
intervening sections 128 comprise the high dielectric constant
material. The fringing electric fields provided by this arrangement
are indicated by the arrows 130.
FIGS. 10 and 11 illustrate another embodiment of a microwave oven
apparatus with the microwave energy from a source 132 fed through
the bottom wall 134 by means of horn radiators 135 and 136. The
load 138 comprising a fowl is supported on a rotisserie arrangement
including a rod member 140 supported by the sidewalls 142 and
actuated by motor means (not shown). In this embodiment the energy
radiated from horn 135 comprises radiated waves 144 with the energy
polarized in a direction to provide the E-vector 146 and the
H-vector 148. These vectors are the orthogonal components of the
plane wave indicated by vector 150. The adjacent horn radiator 136
also provides for the radiation of a polarized wave in the
direction indicated by the vector 150. This radiator, however, is
structured to provide for a different polarization of the energy so
that the E-vector 152 and H-vector 154 are approximately 90.degree.
out-of-phase with the orthogonal distribution from the radiator
135.
In this embodiment the oven enclosure 156 may be larger than
present day microwave ovens since the energy is radiated directly
from the radiators 135 and 136 and the distribution effectively
controlled by the angle of radiation. The energy reflected from the
surfaces of the load 138 as the load rotates are redirected into
the meat because of the reflection from the oven enclosure walls.
In FIG. 11 a microwave plate member 158, similar to that described
in FIG. 9, having alternate dielectric loss material members 160
and 162 is supported on brackets 164. Parallel plate regions 160
represent the high dielectric loss material and the intervening
members 162 represent the low dielectric loss material.
It is evident that numerous other variations, modifications and
alterations may be practiced by those skilled in the art, for
example, it is possible to provide an energy coupling and
transformer browning means with a TM.sub.o1 mode field pattern.
Such a pattern would be suited for baking potatoes. It is intended,
therefore, that the foregoing description of the embodiment of the
invention be considered broadly and not in a limiting sense.
* * * * *