Abstract:
A microwave cooking appliance includes a sheathed resistive electric heating element including an outer portion, an inner portion and a cross-over portion electrically interconnecting the outer and inner portions. The heating element is arranged in a cooking chamber of the microwave cooking appliance in a manner which effects an impedance characteristic of the cooking chamber. The cross-over portion is arranged such that the heating element acts like an RF antenna moving a microwave energy field to portions of the cooking chamber in which low electric fields occur. In this manner, the broil element helps increase the operational efficiency of the cooking applaince by minimizing any hot and cold spots within the cooking chamber.

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 a microwave cooking appliance including a heating element mounted within a cooking chamber such that impedance characteristics of the heating element substantially match impedance characteristic of the cooking chamber in order to reduce the occurrence of high and low electric fields. 
     2. Discussion of the Prior Art 
     The art of cooking is currently undergoing substantial change. It is no longer the norm to have a family member home all day with time to cook and prepare meals. Today, more and more consumers must rush home from work to prepare meals for themselves or for their families. In today&#39;s fast paced society, time is of the essence. The luxury of spending time in preparing a meal is becoming less and less affordable. As such, consumers demand an oven that will cook a meal in less time than conventional ovens, without sacrificing the quality of the prepared food. In order to meet these demands, manufacturers are combining conventional cooking systems with the rapid cook advantages of microwave cooking systems. 
     Cooking appliances utilizing a directed microwave energy field to cook a food item have existed for some time. In such a cooking appliance, food is heated by directing standing microwaves into a cooking chamber where the microwave energy is directed upon the food item to be cooked. As the microwaves are reflected within the chamber, they impinge upon the food item, causing the food item to undergo a cooking process. The nature of the standing waves often results in localized areas of high and low energy fields which, coupled with other factors, cause the food to cook unevenly. This is especially true in larger ovens where the size of the cooking chamber requires a more uniform energy distribution in order to properly cook the food. In addition to confronting the design challenges related to incorporating microwave cooking into larger ovens, combining conventional cooking elements with the microwave systems requires specific considerations relating to the internal geometry of the cooking chamber. 
     For example, introducing conventional electrical heating elements into a microwave oven chamber will impact the impedance characteristics of the chamber. Not only do the microwaves reflect from the chamber walls, but the microwaves would also reflect from the heating elements themselves. Accordingly, the number of modes, and thus hot and cold spots resulting from high and low energy fields, would increase dramatically. 
     Several methods have been proposed in the prior art to address problems with these methods ranging from providing shields for the heating elements in the form of panels, or locating the heating elements below a food support or the like. Each of these arrangements adds to the cost, and can actually detract from the overall efficiency of the system. Other manufacturers choose to ignore the problem entirely in simply incorporating heating elements within the cooking chamber. In any event, despite the prior art arrangements, there exists a need for a microwave cooking appliance employing an electric heating element designed to be incorporated into a cooking chamber wherein the impedance characteristics of the element substantially matches the impedance characteristics of the cooking chamber, thus reducing the effects of moding during a cooking process. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a microwave cooking appliance including an electrical heating element mounted within a cooking chamber. Specifically, the electrical heating element is preferably constituted by sheathed, resistive electrical element defining a broil element having impedance characteristics which, when properly arranged, substantially match the impedance characteristics of the cooking chamber. More specifically, the microwave cooking appliance includes a cooking chamber having interior and exterior upper surfaces, a toroidal-shaped waveguide including a bottom surface having an interior diameter and an exterior diameter, a waveguide input extending from the waveguide, a magnetron arranged on the waveguide input, and a sheathed electric heating element arranged on the interior upper surface of the cooking chamber, wherein the waveguide and electrical heating element includes complementary configurations designed to enhance the overall cooking performance of the appliance. 
     In a preferred embodiment, the sheathed resistive electric heating element includes an outer portion extending about and preferably suspended from the interior upper surface of the cooking cavity, an inner portion extending about an inner diameter of the bottom surface of the waveguide, and a cross-over portion electrically interconnecting the outer and inner portions. In accordance with the invention, the heating element is sized such that the element provide a sufficient amount of heat output necessary to perform a variety of cooking operations. In addition, the element is shaped and located so as to tune the impedance characteristics of the element. In this manner the heating element acts in a manner similar to an RF antenna, directing RF energy within the cooking chamber to areas of low electric field concentrations. 
    
    
     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 an electrical broil element and toroidal waveguide constructed in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a partial perspective view of the toroidal waveguide mounted in accordance with the present invention; 
     FIG. 3 is a cross-sectional view of a portion of the toroidal waveguide of FIG. 2, including a broil element arranged in accordance with the present invention; 
     FIG. 4 is a plan view of an upper oven cavity employed in the wall oven of FIG. 1, particularly illustrating the broil element of FIG. 3 arranged in accordance with one form of the present invention; 
     FIG. 5 is a plan view, similar to that of FIG. 4, but illustrating a broil element constructed in accordance with a second preferred embodiment of with the present invention; and 
     FIG. 6 is a plan view, similar to that of FIG. 4, but illustrating a broil element constructed in accordance with a third preferred embodiment of with the present invention. 
    
    
     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 a broil element  65  which preferably takes the form of a sheathed electric resistive heating element arranged on upper portion  28  as will be set forth more fully below. 
     With reference to FIGS. 2 and 3, 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 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. As shown in FIGS. 3 and 4, hollow interior portion  84  is generally defined by an inner wall portion  85  and an outer wall portion  86 . 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 FIG. 3, 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. 
     The actual use of cooking appliance  2  in connection with microwave cooking is described in co-assigned U.S. patent applications Ser. No. 10/299,918 entitled “TOROIDAL WAVEGUIDE FOR A MICROWAVE COOKING APPLIANCE” filed on Dec. 20, 2002 and incorporated herein by reference. As indicated above, the present invention is particularly directed to the incorporation of broil element  65  and, more specifically, to the particular configuration of broil element  65  within cooking chamber  6 . In one form of the invention as shown in FIG. 4, broil element  65  includes an outer portion  145 , an inner portion  147 , and a cross-over portion  149  which electrically interconnects outer portion  145  and inner portion  147 . As shown, outer portion  145  includes front sections  152  and  153 , opposing side sections  155  and  156 , and rear sections  159  and  160 . In a preferred arrangement, rear sections  159  and  160  terminate in respective terminal ends  163  and  165  that extend through rear portion  33  of cooking chamber  6 . In a manner known in the art, terminal ends  163  and  164  are electrically connected with oven control components (not shown). In addition, an insulating plate  170  is arranged at the interface of rear portion  33  and broil element  65  to isolate broil element  65  from other portions of upper oven  4 . 
     In the embodiment shown, broil element  65  extends directly along and adjacent upper portion  28 . In a preferred form of the invention, outer portion  145  has an associated heat output necessary to perform a pre-heat function, a broil function, and other supplemental oven cavity heating functions. Furthermore, inner portion  147  is dimensioned such that the diameter of inner portion  147  is slightly greater than the diameter of inner wall portion  85  of hollow interior portion  84  of waveguide  67 , while outer portion  145  is larger than a diameter of outer wall portion  86 . 
     While the dimensions of broil element  65  are considered important to the overall arrangement of the present invention, more important is the location of cross-over section  149  relative to microwave launching zone  88 . Incorporating additional structure into a microwave cooking chamber inherently involves several drawbacks, not the least of which is the change in the impedance characteristics of the cooking chamber. In accordance with the invention, broil element  65  is effectively positioned and, more particularly, cross-over portion  149  is arranged so that it acts in a manner similar to an RF antenna, i.e., cross-over portion  149  will direct RF energy to portions of cooking chamber  6  that have lower concentrations of microwave energy. Accordingly, arranging cross-over portion  149  such that it lies substantially along an imaginary line extending through microwave launching zone  88  advantageously functions to tune the impedance characteristics of broil element  65  with the impedance characteristics of cooking chamber  6 . In this manner, broil element  65  helps direct the RF energy within cooking chamber  6 . Accordingly, cross-over portion  149  is preferably arranged centrally along front edge portions  152  and  153 . 
     In another embodiment of the invention as shown in FIG. 5, a broil element  65 ′ is arranged in accordance with another form of the present invention. In a manner similar to that set forth above, broil element  65 ′ includes outer portion  145 ′, inner portion  147 ′, and a cross over portion  149 ′ electrically interconnecting outer portion  145 ′ and inner portion  147 ′. In order to more closely tune the impedance characteristics of broil element  65 ′ with cooking chamber  6 , cross-over portion  149 ′ is positioned in a manner to closely align with microwave launching zone  88 . Accordingly, as shown in FIG. 5, cross-over portion  149 ′ is located at the junction of front portion  152 ′ and side portion  155 ′. In this manner, cross-over portion  149 ′ tunes broil element  65 ′ to more closely match the impedance characteristics of cooking chamber  6 . 
     In accordance with a still further embodiment of the present invention as shown in FIG. 6, a broil element  65 ″ is shown to have an outer portion  145 ″ formed generally in the shape of a circle. As shown, outer portion  145 ″ extends about and encompasses inner portion  147 ″. In a manner similar to that described above, outer portion  145 ″ is interconnected to inner portion  147 ″ through cross-over portion  149 ″. In this preferred form of the invention, cross-over portion  149 ″ is substantially aligned with microwave launching zone  88 . It has been found that circular outer portion  145 ″ and inner portion  147 ″, in combination with the location of cross-over portion  149 ″, presents a close impedance match to cooking chamber  6 . 
     Although described with reference to preferred embodiments 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 launching zone or microwave input section may be located at any point around the toroidal ring cover as long as corresponding changes to the location of the cross-over portion are made. Additionally, the toroidal waveguide can be arranged in other areas with respect to the cooking chamber, including a corresponding change in the location of the broil element. For instance, the waveguide can be mounted on the bottom or rear portions of the cooking chamber. Furthermore, the present invention can be used in combination with a self-clean operation. Certainly, the cooking appliance into which the present invention is incorporated may vary in type, size and model. Particularly, based on these various embodiments, it should be readily apparent that various configurations for the broil element of the invention are available. In general, the invention is only intended to be limited by the scope of the following claims.