Patent Publication Number: US-6667466-B1

Title: Microwave delivery system for a cooking appliance

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention pertains to the art of microwave cooking appliances and, more particularly, to an microwave energy delivery system including launching zone which efficiently delivers a microwave energy field through a waveguide and into an cooking chamber. 
     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 is the molecular excitation of polar molecules, primarily water, present within the food item. However, 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. 
     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 currents supplied by a blower. In any event, all of these methods share a common theme, i.e., to reflect and/or 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 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 a rectangular surface having a first end which is open to the waveguide and a second end onto which a microwave energy source is mounted. 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 toroidal waveguide, and into the cooking chamber. 
    
    
     Additional objects, features and advantages of the present invention will become more readily apparent from the following detailed description of a preferred embodiment when taken in conjunction with the drawings wherein like reference numerals refer to corresponding parts in the several views. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 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; 
     FIG. 2 is a perspective view of the toroidal waveguide and launching system of the present invention; 
     FIG. 3 is a cross-sectional view of the waveguide and launching zone of FIG. 2; and 
     FIG. 4 is an enlarged cross-sectional view of the launching zone of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With initial reference to FIG. 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 FIG. 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, 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 . 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, as well as slide-in and free standing ranges. 
     Further shown in FIG. 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  55  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 FIGS. 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 ring cover  71  having an upper  25  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 ½λ, and the centerline diameter of waveguide  67  is equal to 2λ, where λ 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  87   a  and  87   b . 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  87   a  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 FIG. 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 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 microwaves of a defined wavelength (λ) 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. 
     Referring further to FIG. 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 FIG. 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 ½λ 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, 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  5  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 wavelength. 
     In a preferred form of the present invention, shown in FIG. 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 ½λ and each respective height y is set equal to ¼λ, where λ is the frequency of the microwave energy delivered by magnetron  95 . In a preferred arrangement, launching zone  88  is positioned such that the centerline of launching zone  88  is aligned with the centerline of torus ring cover  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 cover  71 . 
     In a more preferred form of the invention, the interior height of hollow interior portion  84  is set to ½λ in order to tune the microwave energy field as it propagates about torus ring cover  71 . By setting the height of hollow interior portion  84  at ½λ, a maximum energy node is established around the inside and outside of torus ring cover  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 . Thus, as the microwave energy field propagates about torus ring cover  71 , nicrowave 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 FIGS. 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  67  and away from rear reflection wall  92 . In this manner, the microwave energy field couples with top and bottom walls  115  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%. 
     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 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. 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. In stead, the invention is only intended to be limited by the scope of the following claims.