Note: Descriptions are shown in the official language in which they were submitted.
CA 02486771 2004-11-04
MICROWAVE DELIVERY SYSTEM FOR A COOKING
APPLIANCE
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of U. S. Patent
Application Serial No. 10/299,677 entitled "MICROWAVE DELIVERY
SYSTEM FOR A COOKING APPLIANCE" filed on November 20,
2002.
BACKGROUND OF THE INVENTION
1. Field of the Invention
l0 The present invention pertains to the art of microwave cooking
appliances and, more particularly, to an microwave energy delivery
system including a metallic waveguide cover that enhances microwave
energy field distribution in a cooking chamber of the cooking appliance.
CA 02486771 2004-11-04
2. Discussion of the Prior Art
Cooking appliances utilizing a directed microwave energy field to
cook a food item have existed for some time. In general, a cooking
process is performed by heating the food item by directing a standing
microwave energy field into an oven cavity such that the microwave
energy field reflects about the oven cavity and impinges upon the food
item. As the microwave energy fields impinge upon the food item, the
fields are converted into heat through two mechanisms. The first heating
mechanism is caused by the linear acceleration of ions, generally in the
form of salts, present within the food item. The second heating
mechanism is the molecular excitation of polar molecules, primarily
water, present within the food item. Regardless of the mechanism, the
nature of the standing waves results in localized areas of high and low
energy which cause the food to cook unevenly. This is especially true in
larger ovens where the size of the cavity requires a more uniform energy
distribution in order to properly cook the food. To attain an even, or
uniform energy distribution, the microwave energy must be introduced
into the oven cavity in a manner which creates a constructive standing
wave front which will propagate about the oven cavity in a random
fashion.
Various methods of directing microwaves into cooking chambers
to minimize hot and cold areas within a food item have been proposed in
the prior art. These methods range from altering the pattern of the
standing waves by varying the frequency of the microwave energy field,
to incorporating a stationary mode stirrer which simulates a change in the
geometric space of the cooking chamber.
2
CA 02486771 2004-11-04
Methods of changing the wave pattern also include the
incorporation of a rotating blade stirrer which functions to reflect
microwave energy into a cooking cavity in various patterns.
Traditionally, stirrers have been located in various points in the
microwave feed system, ranging from adjacent to a microwave energy
source, to a position within the cooking chamber itself. Some stirrers
include various openings which are provided to disperse the standing
waves, and others have various surface configurations designed to reflect
the standing waves. Stirrers are either driven by a motor, or by air
to currents supplied by a blower. In any event, all of these methods share a
common theme, i.e., to reflect andlor deflect the microwave energy into a
cooking cavity such that a uniform distribution of standing wave patterns
can be achieved.
Other methods include modifying the structure of the waveguide
itself. Waveguide designs include cylinders, square boxes, and a variety
of other configurations, each having an exit window through which the
microwave energy can pass. While, these designs may cause the standing
waves to interfere with one another such that the wave pattern was
randomized, substantial energy is typically lost with such an arrangement.
Still other methods are directed to rotating or moving the food
being cooked within the cooking chamber. Ovens employing this
method, position the food on a rotatable platter which is rotated through
the standing wave patterns such that the food is more uniformly exposed
to the microwaves. While these methods are fine for smaller ovens, they
are hardly practical for larger conventional ovens where space is more of
a concern. As oven cavities have grown in size and microwave
3
CA 02486771 2004-11-04
technology has been combined into conventional or convection ovens, the
uniform distribution of the standing waves has become of even greater
concern. For this reason, manufacturers have modified their designs to
include multiple magnetrons, multiple stirrers, and motor driven, variable
speed stirrers, all of which were intended to create a random wave pattern
thought to be of a more uniform character. Certainly, the mechanisms
which serve to defect the microwave energy field, e.g., stirring fans and
turntables, add to the complexity of designs and introduce multiple failure
points, thus reducing the service life of such appliances. Furthermore, in
an age where energy consumption is of a concern, the need for an energy
efficient cooking appliance is desired.
Based on the above, there exists a need for a microwave delivery
system which will direct a uniform standing wave pattern into an cooking
chamber in a manner that reduces the complexity of system components,
minimizes energy losses within a waveguide, and provides a uniform,
maximum energy field source to the cooking chamber.
SUMMARY OF THE INVENTION
The present invention is directed to a microwave cooking appliance
including a cooking chamber, and a microwave energy delivery system
including an annular, toroidal-shaped waveguide, a launching zone, and a
magnetron. In one form of the invention, the waveguide includes an
upper surface, a hollow interior portion exposed to the cooking chamber,
and a circular bottom surface. The launching zone serves as an interface
between the magnetron and the waveguide. The launching zone includes
4
CA 02486771 2004-11-04
a first end which is open to the waveguide and a second end onto which a
microwave energy source is mounted. Actually, the launching zone is
rectangular in cross-section and includes top, bottom, and opposing side
walls that define a passage through which the microwave energy field
travels. In one preferred form of the invention, the bottom wall is aligned
with the bottom surface of the waveguide. The microwave energy source
takes the form of a magnetron including an antenna which extends into
the launching zone. Upon activation of the magnetron, a microwave
energy field is generated in the launching zone, directed through the
l0 toroidal waveguide, and into the cooking chamber.
In further accordance with the invention, the microwave delivery
system includes a waveguide cover that shields the hollow interior
portion of the toroidal-shaped waveguide from cooking byproducts
generated in the cooking chamber during a cooking process. Preferably,
1 s the waveguide cover includes a plurality of microwave transparent
regions that correspond to an associated plurality of microwave
transparent regions arrayed about the bottom surface of the waveguide.
Most preferably, the microwave transparent regions are aligned so as to
coincide with high energy peaks of the microwave energy field. In the
20 most preferred form of the invention the waveguide cover is constituted
by a metallic conductor. The metallic conductor includes an exposed,
preferably aluminum finished surface which enhances the microwave
energy field distribution in the cooking chamber.
Additional objects, features and advantages of the present
25 invention will become more readily apparent from the following detailed
description of preferred embodiments when taken in conjunction with the
CA 02486771 2004-11-04
drawings wherein like reference numerals refer to corresponding parts in
the several views.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a perspective view of a combination
microwave/convection wall oven including a toroidal waveguide and
launching zone constructed in accordance with the present invention;
Figure 2 is a perspective view of the toroidal waveguide and
launching system of the present invention;
Figure 3 is a cross-sectional view of the waveguide and launching
1 o zone of Figure 2;
Figure 4 is an enlarged cross-sectional view of the launching zone
of Figure 3;
Figure S is a cross-sectional view of the waveguide and launching
zone constructed in accordance with a second embodiment of the present
invention; and
Figure 6 is a partial, lower left perspective view of an interior of a
cooking chamber illustrating a waveguide cover constructed in
accordance with the present invention.
CA 02486771 2004-11-04
DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENTS
With initial reference to Figure 1, a microwave cooking appliance
constructed in accordance with the present invention is generally
indicated at 2. Although the form of cooking appliance 2 in accordance
with the present invention can vary, the invention is shown in connection
with cooking appliance 2 depicted as a wall oven. More specifically, in
the embodiment shown, cooking appliance 2 constitutes a dual oven wall
unit including an upper oven 4 having upper cooking chamber 6 and a
lower oven 8 having a lower cooking chamber 10. In the embodiment
shown, upper oven 4 is adapted to perform a rapid cook or combination
microwave/convection cooking process, and lower oven 8 is provided to
perform a standard convection and/or radiant heat cooking operation. As
shown, cooking appliance 2 includes an outer frame 12 for supporting
upper and lower cooking chambers 6 and 10.
In a manner known in the art, a door assembly 14 is provided to
selectively provide access to upper cooking chamber 6. As shown, door
assembly 14 is provided with a handle 15 at an upper portion 16 thereof.
Door assembly 14 is adapted to pivot at a lower portion 18 to enable
selective access to within cooking chamber 6. In a manner also known in
the art, door 14 is provided with a transparent zone 22 for viewing
cooking chamber 6 while door 14 is closed.
As best seen in Figure 1, cooking chamber 6 is defined by a bottom
portion 27, an upper portion 28, opposing side portions 30 and 31, and a
rear portion 33. Bottom portion 27 is preferably constituted by a flat,
CA 02486771 2004-11-04
smooth surface designed to improve the cleanability, serviceability, and
reflective qualities of cooking chamber 6. In the embodiment shown,
arranged on rear portion 33 is a convection fan 37 having a perforated
cover 39 through which heated air can be withdrawn from cooking
chamber 6. Heated air is re-introduced into cooking chamber 6 through
vents 42 and 43 arranged on either side of fan 37. In addition, a broil
element 45, shown in the from of a sheathed, resistive electric heating
element, is provided at upper portion 28 of cooking chamber 6. Although
cooking appliance 2 is depicted as a wall oven, it should be understood
that the present invention is not limited to this model type and can be
incorporated into various types of oven configurations, e.g., cabinet
mounted ovens, a.s well as slide-in and free standing ranges.
Further shown in Figure 1, cooking appliance 2 includes an upper
control panel 50 incorporating first and second rows of oven control
button rows 52 and 53. Control buttons 52 and 53, in combination with a
numeric pad 5 S and a display 57, enable a user to establish particular
cooking operations for upper and lower ovens 4 and 8 respectively. Since
the general programming and operation of cooking appliance 2 is known
in the art and does not form part of the present invention, these features
will not be discussed further here. Instead, the present invention is
particularly directed to the incorporation and construction of a microwave
energy delivery system for delivering a microwave energy field into
cooking chamber 6 as will be detailed fully below.
With reference to Figures 2-4, a waveguide 67 is shown mounted
on an exterior upper portion 69 of cooking chamber 6. More specifically,
waveguide 67 includes an annular toroidal or torus ring 71 having an
8
CA 02486771 2004-11-04
upper surface 73 defining a central depression 75, and a bottom surface
80. In a preferred form of the invention, waveguide 67 further includes a
hollow interior portion 84, defined between inner and outer walls 85 and
86, having a defined torus ring or cross-sectional diameter and a defined
centerline diameter. Waveguide 67 is preferably formed from coated
aluminum which provides enhanced reflective qualities, while also
decreasing any IR emissivity. As such, energy loses due to the absorption
of microwave energy are minimized. In a preferred arrangement, the
torus ring diameter of waveguide 67 is set equal to '/2 ~,, and the centerline
diameter of waveguide 67 is equal to 2~,, where ~, is defined as the
wavelength of the microwave energy field transmitted into waveguide 67.
In a preferred form of the present invention, hollow interior portion
84 and central depression 75 contain a quantity of insulation material 87a
and 87b. In general, insulation material 87 may be of any type of known
insulation provided that the material is transparent or substantially
transparent to microwave energy. Examples of acceptable types of
insulation material are standard spun glass, fiberglass insulation, ceramic
fiber insulation, or the like. The addition of insulation material 87a to
hollow interior portion 84 limits heat transfer losses to approximately the
same level as an oven simply covered with an insulation blanket, but does
not require insulation to be added over the waveguide. In this manner,
cooking appliance 2, if required, can be used in a more space restrictive
application.
As best shown in Figure 2, a launching zone 88 is provided which
includes a first end defining an exit 90 opening into waveguide 67, and a
second, terminal end 92 which constitutes a rear, microwave reflecting
9
CA 02486771 2004-11-04
wall. Mounted on an upper portion of terminal end 92 is a magnetron or
microwave emitter 95. In a manner known in the art, magnetron 95 emits
microwave or RF energy of a defined wavelength (~,) into launching zone
88. In a preferred configuration, magnetron 95 emits microwave energy
at a wavelength of 2.45 GHz. However, it should be noted that
waveguide 67 of the present invention is adaptable to any acceptable
wavelength used for cooking.
Refernng further to Figure 2, arranged about a front portion of
waveguide 67 are a plurality of inlet openings 98. More specifically, inlet
openings 98 are positioned to allow a flow of cooling air to enter interior
portion 84. Additionally, a plurality of exhaust openings 99 are arranged
on a rear portion of waveguide 67, adjacent to launching zone 88, to
allow heated air to escape from interior portion 84. In this manner,
waveguide 67 also serves as an air duct, further eliminating the amount of
insulation required over cooking chamber 6. Inlet openings 98 and
exhaust openings 99 are sized and positioned such that the reflected
microwave energy field will not escape from interior portion 84.
As best seen in Figure 2, a plurality of cavity excitation ports 103
are arranged about bottom surface 80 of waveguide 67. Specifically,
cavity excitations ports 103 are located about bottom surface 80 at each
point where a maximum energy node will occur. As such, in the most
preferred form of the invention, three equally spaced excitation ports are
positioned at'/Z ~, points located about bottom surface 80.
A particularly important aspect of the present invention is the
design of rectangular launching zone 88. In a manner known in the art,
14
CA 02486771 2004-11-04
magnetron 95 includes an antenna 108, from which the microwave
energy field emanates. In accordance with a preferred embodiment,
antenna 108 extends into launching zone 88 and is preferably positioned
between hollow interior portion 84 and the rear reflecting wall 92. In a
manner also known in the art, magnetron 95 emits microwaves of a
defined wavelength into launching zone 88 which are subsequently
delivered into waveguide 67. In a preferred configuration, magnetron 95
emits microwave energy at a wavelength of 2.45 GHz, however, it should
be noted that the waveguide of the present invention is adaptable to any
1 o wavelength.
In a preferred form of the present invention, shown in Figure 4,
launching zone 88 includes an interior metallic surface 112 defined by
opposing upper and lower walls 115 and 116 each having a respective
width x, and opposing side walls 117 and 118 each having a respective
height y. In a manner similar to that of hollow interior portion 84,
interior metallic surface 112 is formed from coated aluminum. In a more
preferred form of the invention, each respective width x is set equal to 'h
~, and each respective height y is set equal to 1/ ~,, where ~, is the
frequency of the microwave energy delivered by magnetron 95. In a
2o preferred arrangement, launching zone 88 is positioned such that the
centerline of launching zone 88 is aligned with the centerline of torus ring
71, however, other arrangements are possible without departing from the
scope of the present invention. For example, one acceptable alternative
locates launching zone 88 perpendicular to torus ring 71.
In accordance with another embodiment of the present invention,
lower wall 116 of launching zone 88 is aligned with bottom surface 80 of
11
CA 02486771 2004-11-04
waveguide 67 as represented in Figure S. In this manner, a continuous,
direct conducting surface is established that provides a path for the
microwave energy to propagate from antenna 108 of magnetron 95. This
direct conducting surface further enhances the coupling of the microwave
or RF energy field by providing a continuous pathway through launching
zone 88, into and around the centerline of toroidal-shaped waveguide 67,
before launching the energy field into cooking chamber 6. This design
causes a mixing action to occur within waveguide 67 that generates
multiple high energy nodes that establish constructive standing waves
inside cooking chamber 6. The constructive standing waves illuminate
cooking chamber 6 with an extremely uniform energy field having a
variation of less than approximately 1/z %. With this overall construction,
a uniform cooking environment is achieved in cooking chamber 6 without
requiring any additional energy field modification, such as through the
use of a mode stirrer.
In the most preferred form of the invention, the interior height of
hollow interior portion 84 is set to '/2 ~, in order to tune the microwave
energy field as it propagates about torus ring 71. By setting the height of
hollow interior portion 84 at '/2 ~,, a maximum energy node is established
2o around the inside and outside of torus ring 71. Specifically, the
microwave energy field traveling from launching zone 88, through
waveguide 67 into cooking chamber 6 is tuned for maximum uniformity.
As such, further modification of the microwave energy field such as the
incorporation of a mode stirrer, is not required.
In another form of the invention, a plurality of microwave windows
135 are positioned below a respective one of cavity excitation ports 103.
12
CA 02486771 2004-11-04
Thus, as the microwave energy field propagates about torus ring 71,
microwave energy is transmitted from waveguide 67 through microwave
windows 135 and into cooking chamber 6 whereupon the microwave
energy impinges upon the food item to perform a cooking operation. As
the microwave energy is released through cavity excitation ports 103 into
cooking chamber 6, constructive and destructive wave interferences will
occur. In this manner, the microwave energy field is focused, and caused
to move about cooking chamber 6 delivering a high, uniform energy
density to the food item.
In a more preferred form of the invention, a waveguide cover 140
is arranged between waveguide 67 and microwave windows 135. In
general, waveguide cover 140 is designed to withstand the highest oven
operating temperatures in addition to being transparent to microwave
energy. As such, microwave cover 140 can be formed from Pyrex glass,
ceramic sheets, mica, silicon mica or the like. The incorporation of
waveguide cover 140 prevents cooking byproducts such as grease, oil,
fats and the like from entering waveguide 67.
Reference will now be made to Figures 1-4 is describing a
preferred method of operation of cooking appliance 2. Prior to
initializing a cooking operation, a food item is placed into cooking
chamber 6. Control 52 is operated either individually, or in conjunction
with control 55 to select a desired cooking operation. Upon activation,
magnetron 95 begins to emit a microwave energy field from antenna 108
into launching zone 88. The microwave energy field then impinges upon
the interior metallic surface 112 of launching zone 88 located adjacent to
antenna 108. The microwaves subsequently propagate toward waveguide
13
CA 02486771 2004-11-04
67 and away from rear reflection wall 92. In this manner, the microwave
energy field couples with top and bottom walls 11 S and 116 and side
walls 117 and 118 thus enhancing the transmission of energy from
launching zone 88 to waveguide 67. Experience has shown that the
energy coupling created within launching zone 88 significantly increases
the efficiency of the microwave delivery system. In some cases, the
efficiency level can rise as much as 200%.
Reference will now be made to Figure 6 in describing a waveguide
cover 200 constructed in accordance with a further embodiment of the
l0 present invention. As shown, waveguide cover 200 is arranged above
broil element 45 on upper portion 28 of cooking chamber 6. Preferably,
waveguide cover 200 defines a smooth, flat metallic conductor surface
205 through which extend microwave windows 135. Although windows
135 are depicted as openings in conductor surface 205, it should be
understood that windows 135 could be constituted by regions formed on
conductor surface 205 that are transparent to microwave or RF energy
without creating actual openings in waveguide cover 200. As set forth
above, microwave windows 135 correspond to openings 103 in bottom
surface 80 of waveguide 67. Openings 103 are positioned along a
centerline of toroidal-shaped waveguide 67 and are aligned at each point
where a maximum energy node or high energy peak will occur. In
accordance with this preferred form of the invention, metallic conductor
surface 205 has an aluminum finish that provides maximum reflectivity
of the microwave or RF energy waves, in addition to increasing the
thermal resistance, in cooking chamber 6. Thus, waveguide cover 200
further improves the energy field distribution within cooking chamber 6
so that food items are exposed to a more uniform cooking process.
14
CA 02486771 2004-11-04
In addition to improving microwave performance, waveguide cover
200 contributes to the performance of broil element 45. During a broil
operation, broil element 45 is activated to direct radiant heat energy onto
an exposed surface of a food item. Due to the structure of broil element
45, a portion of heat energy generated is actually radiated in a direction
away from the food item. That is, some portion of the heat energy is
directed upward toward upper portion 28 of cooking chamber 6. This
portion of the heat energy either dissipates or simply contributes to the
overall heating of oven chamber 6. In either case, the heat energy is not
1 o directed onto the exposed surface of the food item as desired for by the
broil operation. However, in accordance with another aspect of the
present invention, the aluminum finish provided on metallic surface 205
operates to reflect the portion of the heat energy directed upward from
broil element 45 back onto the food item. In this manner, the overall
efficiency of the broil operation is improved.
Based on the above, it should be readily apparent that the invention
provides for an improved microwave energy delivery system, in the form
of a toroidal waveguide and microwave energy launching system that
creates a uniform cooking environment for a food item. In any event, it
2o should be understood that although described with reference to a
preferred embodiment of the invention, it should be readily understood
that various changes and/or modifications can be made to the invention
without departing from the spirit thereof. For instance, the invention
while described in terms of a microwave/convection wall oven, can be
included in a combination oven range or self standing microwave oven
without departing from the scope of the present invention. Also, while
CA 02486771 2004-11-04
the reflective finish provided on waveguide cover 200 is established
through the use of aluminum, a variety of other reflective finishes could
be employed. Finally, it should be recognized that the use of terms such
as top, bottom, left and right have been presented for illustrative purposes
only and should not limit the scope of the present invention. Instead, the
invention is only intended to be limited by the scope of the following
claims.
16
