Abstract:
A microwave delivery system for a cooking appliance includes a toroidal waveguide having a ring diameter twice that of the wavelength of the microwaves produced by a magnetron, and a cross-sectional diameter equal to one-half of the produced microwaves. The waveguide includes a plurality of cavity excitation ports through which the microwaves enter a cooking cavity. The delivery system further includes a rotating stirrer positioned within the waveguide. The stirrer includes a plurality of openings which become aligned with the cavity excitation ports when the stirrer is rotated to create a uniform, high energy pattern of microwave energy to enter the cooking cavity. Finally, the waveguide includes a plurality of openings along spaced portions thereof to allow a flow of air through the waveguide.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention pertains to the art of cooking appliances and, more particularly, to a microwave energy delivery system including a toroidal waveguide which efficiently delivers a substantially uniform microwave energy field into a cooking chamber. 
     2. Discussion of Prior Art 
     Cooking appliances utilizing directed microwave energy fields to cook food items 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 field impinges upon the food item, the field is converted into heat through two mechanisms. The first, ionic heating is caused by the liner 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 resulting from the existence of high and low energy fields 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 change in the geometric space of the cooking chamber. Methods of changing the wave pattern have also included the incorporation of a rotating blade stirrer which functions to deflect 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 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 of controlling the standing waves include modifying the structure of the waveguide itself. The prior art provides examples of waveguides shown as cylinders, square boxes, and a variety of other configurations designed to cause the standing waves to interfere with one another in a manner which results in a randomized wave front such that a maximum energy field is directed into the cooking chamber. Other designs include matching the dimensions of the waveguide to the wavelengths of the standing wave pattern. However, these designs, while effective to a point, have failed to adequately address the problem of energy loss due to energy absorption on the waveguide surface. 
     As the desire to increase the sizes of oven cavities has risen, 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 are intended to create a random wave pattern thought to be of a more uniform character. Still other designs include structure for rotating or moving food within the cooking chamber. Ovens employing this method, position the food on a 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. Certainly, in an age where energy consumption is of particular 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 a cooking chamber in a manner that minimizes energy losses within a waveguide, yet provides a uniform, maximum energy field to the cooking chamber. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a microwave cooking appliance including a toroidal-shaped waveguide preferably having a ring diameter equal to twice the standing wavelength, and a cross-sectional diameter equal to one-half the standing wavelength. The design of the waveguide causes a standing wave to impinge upon an inner surface of the waveguide at points of zero energy such that energy absorbed by the waveguide is minimized. Additionally, the design of the ring diameter causes constructive interferences within the waveguide, thereby a high energy node about the circumference of the waveguide. Furthermore, a plurality of cavity excitation ports are arranged about the bottom portion of the waveguide. 
     In accordance with a preferred embodiment, the microwave delivery system of the present invention further includes a mode stirrer having a plurality of openings evenly spaced about the periphery of the stirrer. Specifically, when rotated, the openings operate as shutters. As the openings align with the cavity excitation ports, passages are created to allow the microwave energy field into the cooking chamber. The operation of the stirrer creates a uniform pattern of microwave energy to be directed onto a food item placed within the cooking chamber. 
    
    
     In any event, additional objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention, 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 constructed in accordance with the present invention; 
     FIG. 2 is a perspective view of the toroidal waveguide mounted in accordance with the present invention; 
     FIG. 3 is a partial cross-sectional view of the toroidal waveguide of FIG. 2; and 
     FIG. 4 is a cross-sectional view of the toroidal waveguide of FIG. 3 inverted. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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 waveguide  67  for delivering a microwave energy field into cooking chamber  6 , which provides for at least a portion of the cooking operation, as will be detailed fully below. 
     With reference to FIGS. 2-4, 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 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  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 . 
     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 . 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 FIGS. 3 and 4, 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 . 
     Referring back to FIGS. 2 and 3, a stirring plate  110  is shown rotatably mounted within interior portion  84 . In a preferred form of the invention, a plurality of openings  115  are arranged about stirring plate  110 . In the most preferred form of the invention, the number of openings  115  correspond to the number of cavity excitation ports  103 . Stirring plate  110  is driven by a motor  120  arranged within central depression  75 , with motor  120  being drivingly connected to stirring plate  110  through shaft  123 . Shaft  123  is formed from a dielectric material such that it does not interfere with the microwave energy field. Alternatively, in place of using a dielectric material, shaft  123  can be grounded to cooking appliance  2  to avoid interference with the microwave energy field. 
     Reference will now be made to FIGS. 1-4 is describing a preferred method of operation of cooking appliance  2 . Although the described method will be focused on the microwave aspects of the cooking process, it should be understood that the present invention is equally applicable to a combination microwave/convection oven and can even include a radiant heating feature. Prior to initializing a cooking operation, a food item is placed within cooking chamber  6 . Control  52  is operated, either individually or in combination with control  55 , to select a desired cooking operation. Upon activation, magnetron  95  begins to emit a microwave energy field into waveguide  67  through launching zone  88 . Most preferably, motor  120  begins to rotate stirring plate  110  to influence the microwave energy field emitted by magnetron  95 . In general, stirring plate  110  acts to control, shift and modify the microwave energy field. As stirring plate  110  rotates, openings  115  come into alignment with cavity excitation ports  103 . In this manner, cavity excitation ports  103  are intermittently opened and closed such that one, two or all three of ports  103  are opened at any given time. The opening pattern of excitation ports  103  influences the pattern of microwave energy as the energy field is transmitted into cooking chamber  6  to cook the food item. 
     In a preferred form of the invention, a plurality of microwave windows  135  are positioned below the cavity excitation ports  103  respectively. Therefore, 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 undergoing the selected cooking operation. As the microwave energy is released through each cavity excitation port  103  into cooking chamber  6 , constructive wave interferences are generated within cooking chamber  6 . In this manner, the microwave energy field is caused to move about cooking chamber  6  in such a random fashion so as to establish a highly uniform RF energy field. Accordingly, the food item is subjected to a uniform cooking process, such that localized hot and cold spots are substantially eliminated. 
     In further accordance with the invention, a waveguide cover or protective plate  140  is arranged between waveguide  67  and microwave windows  135 . In the embodiment shown, waveguide cover  140  is designed to withstand the highest temperatures developed within the oven. Additionally, waveguide cover  140  is formed from a material which is transparent to microwave energy. Examples of acceptable materials for waveguide cover  140  are: Pyrex glass, ceramic sheets, mica, silicon mica and the like. However, it should be understood that a wide variety of other materials are also acceptable. In general, microwave cover  140  functions to prevent cooking byproducts, such as grease, oil, fats and the like, released from the food item during the cooking process from entering waveguide  67 . 
     Based on the above, it should be readily apparent that the present invention provides an microwave energy delivery system in the form of a toroidal waveguide that creates a uniform cooking environment for a food item. 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. Particularly, 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 be considered to limit the scope of the present invention. For example, although waveguide  67  has be described as being arranged above cooking chamber  6 , waveguide  67  could be repositioned, such as on bottom portion  27  or rear portion  33  without departing from the invention. Instead, the invention is only intended to be limited by the scope of the following claims.