Patent Document

CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 61/898,569, filed Nov. 1, 2013, titled “Category M Microwave Oven Door Seal,” the entire contents of which are hereby incorporated by reference. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    Embodiments of the present disclosure relate generally to microwave oven door seal configurations that are designed to reduce power leakage from microwave ovens. The concepts provided may find particular use on-board aircraft or other passenger transport vehicles that have various types of communication equipment that operate at a similar frequency as microwave ovens, and for which interference should be reduced or eliminated. 
       BACKGROUND 
       [0003]    In microwave oven design, the ability to prevent microwave energy leakage can be a primary focus. First, leakage should be prevented in order to protect users from exposure to the microwave energy. Second, leakage should be prevented so as not to interfere with communication devices working in the same bands. For example, Wi-Fi and microwave oven manufacturers are required by the Federal Communication Commission (FCC) to operate within any of a finite number of allocated frequency bands. These bands may be referred to as ISM (industrial, scientific, and medical) radio bands. Based on a variety of factors, the band that makes the most sense for Wi-Fi and microwave ovens is the 2.4-2.5 GHz band. This means that the frequency of the microwave oven and the frequency of the LAN (local area network) communication use the same ISM band of 2.45 GHz. The electromagnetic noise generated from the microwave oven can create a potential interference with the wireless LAN communication Wi-Fi equipment, causing communication errors. The powerful emissions of microwave ovens can create electromagnetic interference that disrupts radio communications using the same frequency. This can be a particular problem on-board aircraft, where the need for internet services on-board has increased. 
         [0004]    In an effort to provide compatibility between microwave ovens and communication devices operating within the same band, there have been attempts to contain the microwave power to a level that is low enough that it does not cause interference. The Radio Technical Commission for Aeronautics (RTCA) document DO-160 provides emission limits (for all equipment, not specific to microwave ovens) that have been determined to ensure interference free operation between devices. The “Category M” limit is the strictest limit within the 2.4-2.5 GHz frequency range, and allows a field strength of only 68 dBuV/m at a one meter distance from the unit. 
         [0005]    Microwave ovens are generally designed to meet a requirement for human safety, which has been defined internationally as a power density of less than 5 mW/cm 2  at a distance of 5 cm from any point on the unit. That limit, if integrated around the door seal and translated to a one meter distance, and converted from power density to field strength exceeds the Category M limit by many orders of magnitude. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a side schematic view that shows a multi-stage door seal. 
           [0007]      FIG. 2  is a top perspective view of a cavity seal for use with a multi-stage door seal. 
           [0008]      FIG. 3  is a side cross-sectional view of the cavity seal of  FIG. 2 . 
           [0009]      FIG. 4  is a top perspective view of a door seal for use with a multi-stage door seal. 
           [0010]      FIG. 5  is a side cross-sectional view of the door seal of  FIG. 4 . 
           [0011]      FIGS. 6 and 6A  are side cross-sectional views that show a cavity seal and door seal in a partially open position. 
           [0012]      FIG. 7  is side cross-sectional view that shows an alternate multi-stage door seal with angled walls. 
           [0013]      FIG. 8  is a side cross sectional-view that shows the multi-stage door seal of  FIG. 7  in a partially open position. 
           [0014]      FIG. 9  is a side cross-sectional view of an alternate multi-stage door seal is an partially open position. 
           [0015]      FIG. 10  is a side cross-sectional view of the seal of  FIG. 9  in a closed position. 
           [0016]      FIG. 11  is a top perspective view that shows the seal of  FIGS. 9 and 10  in place on a microwave oven cavity. 
           [0017]      FIG. 12  is a top perspective view that shows a close-up view of the seal of  FIGS. 9 and 10  in place on a microwave oven cavity. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Measurements of typical microwave ovens confirm that units emitting a power density of less than 1% the maximum safety limit still emit enough power that the field strength at one meter greatly exceeds the Category M limit. In order to reduce the field strength emitted to a level below Category M requires a reduction in power leakage of at least about 50 dB, or 100,000 times. 
         [0019]    One of the greatest sealing challenges for designing a microwave for aeronautical use (or other vehicle that should comply with Category M) is the microwave oven door seal. Microwave energy will not transmit through solid metal. However, the door must open and close for placement of food in the cavity. The working parts of the door and its required ease of use (e.g., it must be relatively easy for a user to open and close) add challenges to reducing power leakage by such a great amount. 
         [0020]    Some attempted designs have failed because they require extreme door closure force. Such designs often use multiple conductive gaskets. The resulting force that is required to overcome the conductive gaskets is so great that to in order to close the door, power is required from the aircraft. Additionally, because of the door strength required by the high closure forces and because of the multiple, large conductive gaskets, door and the interface flange are heavy, which is undesirable in an aircraft application. Further, the effectiveness of conductive gaskets is dependent upon continuous, low resistance contact. The continuous contact can be rapidly degraded by contamination with food oils, grease, particles, dust, and so forth. 
         [0021]    Accordingly, it is desirable to provide a microwave oven door seal that does not rely solely on conductive gaskets. It is also desirable to provide a microwave oven door seal that is lightweight and can be closed without aircraft power. 
         [0022]    Embodiments of the present disclosure provide a multi-stage door seal  10 . The components of the multi-stage door seal  10  include a choke seal  12 , a single conductive seal  14 , and one or more absorbent material stages  16 . Referring now to  FIG. 1 , there is shown a microwave oven cavity  18  and a cavity seal  20 .  FIG. 1  also shows a microwave oven door  22  and the related door seal  24 . The collective cavity seal  20  and the door seal  24  cooperate with one another so as to form the multi-stage door seal  10 .  FIGS. 2-3  show a cavity seal  20 .  FIGS. 4-5  show a door seal  24 .  FIGS. 1 and 6  show cooperation between the cavity seal  20  and the door seal  24 . 
         [0023]    Referring now to  FIG. 2 , the cavity seal  20  has an inner ledge  26  and a first wall  28 . Inner ledge  26  and first wall  28  help define a space  30  into which the choke  12  can fit. The cavity seal  20  may also have a second wall  32  and a third wall  34 . The second wall  32  may be a stand-alone wall that forms a flange-like structure between first wall  26  and third wall  34 . The third wall  34  may be the inner edge of the cavity perimeter  36 . A first groove  38  may be formed between the first wall  28  and the second wall  32 . A second groove  40  may be formed between the second wall  32  and the third wall  34 . These walls and grooves are also shown in the cross-sectional view of  FIG. 3 . These walls and grooves create a series of bends that microwave energy would have to traverse in order to exit the inner cavity  18  to the outside. 
         [0024]    Referring now to  FIG. 4 , the door seal  24  includes a base  42  that forms front surface of the door. At an inner-most part of the base  42  is a window attachment portion  44 . This is the area where an inner plate  48  may be installed. The inner plate may include a center section  101 , which may include a window so that the user can view the microwave contents. Alternately, center section  101  may be windowless (blank plate). In either case, plate  48  (with or without window) may extend outward, beyond the attachment portion  44  and form one wall  102  of choke  12 . This is the area where a microwave window may be installed so that the user can view the microwave contents. 
         [0025]    The door seal  24  may also include a microwave choke  12 . The choke  12  is defined in part by a raised wall  46  on the door seal  24  and the base  42  of the door seal  24 . As shown in  FIGS. 1 and 6 , the choke is also defined in part by wall portion  102  the plate  48  that covers the window opening  50  and the inner ledge  26  of the cavity seal  20 . Most microwave ovens available in the market have choke structures that attenuate or prevent leakage of microwave energy from the joint between the door and the cavity. The choke seal  12  generally creates a U or box-shaped area  30  where microwave energy may travel. Microwave energy emitted travels along the choke walls and reflects back upon itself, changing its impedance. This can set up an impedance mismatch, which greatly attenuates the perimeter leakage. However, some signal level energy may escape this first choke seal  12 . Accordingly, further seal elements are outlined below. 
         [0026]    Referring back to the door seal  24  of  FIG. 4 , adjacent to the raised wall  46  is an inner groove  52 . Inner groove  52  is positioned between the raised wall  46  and an inner door flange  54 .  FIG. 4  also illustrates an outer door flange  56 . Between the outer door flange  56  and the inner door flange is an outer groove  58 . These flanges and grooves are also shown in the cross-sectional view of  FIG. 5 . These flanges and grooves create a series of bends that microwave energy would have to traverse in order to exit the inner cavity  18  to the outside. 
         [0027]    As shown in  FIG. 3 , a single conductive gasket seal  14  may be provided on the cavity seal  20 . In one example, the conductive gasket seal  14  may be provided along an inner surface  60  of the first wall  28 . The conductive gasket seal  14  may be one or more copper fingers that press between the door and the cavity wall in order to create a short circuit and prevent escape of energy. The conductive gasket seal  14  may be an aluminum, steel, or stainless steel strip. The conductive gasket seal  14  may be a conductive fabric wrapped around an open cell foam inner core. The conductive gasket seal  14  may be any other type of conductive gasket seal. It is generally intended that only a single conductive gasket seal be used, as one of the drawbacks of such seals is that they require a good deal of force to open. Using more than one conductive seal can result in a door that requires aircraft power to open or at the very least, requires a great deal of user force. This would not lead to a microwave with an elegant look and feel. However, it has been found that use of a single conductive seal can improve the leakage levels, while requiring only a relatively low closure force. 
         [0028]    An absorbent material stage  16  is also provided.  FIG. 6A  shows a blown up view of the absorbent material stage  16  of  FIG. 6 . The absorbent material stage  16  may be positioned toward an outer-most edge of both the cavity seal  20  and the door seal  24 . However, it should be understood that the various seal options  12 ,  14 , and  16  may have their locations interchanged if desired. The absorbent material stage  16  provides one or more stages of absorbent material  62  arranged within a series of bends. This stage  16  may be formed by features on the cavity seal  20  that cooperate with features on the door seal  24 . 
         [0029]    As shown in  FIG. 6A , in one example, absorbent material  62   a  may be positioned in the first groove  38  of the cavity seal  20 . Absorbent material  62   b  may be positioned in the second groove  40  of the cavity seal  20 . Absorbent material  62   c  may be positioned in the inner groove  52  of the door seal  24 . Absorbent material  62   d  may be positioned in the outer groove  58  of the door seal  24 . Although four stages of absorbent material are shown and described, it should be understood that more or fewer stages may be used. For example, each of the cavity seal  20  and the door seal  24  may have additional walls or flanges, such that additional grooves are created. Alternatively, for example, each of the cavity seal  20  and the door seal  24  may have fewer walls or flanges, such that only one groove in each is created. 
         [0030]    The absorbent material stage  16  provides multiple absorbent material components  62  along a convoluted path. The general goal is that the absorbent material stage  16  helps absorb any energy that is not attenuated by the choke  12  or shorted out by the conductive gasket  14  (not shown in  FIG. 6A  for ease of review). In order for such escaping energy to exit the microwave oven entirely, it must now traverse the series of turns created by described walls, flanges, and grooves. Whatever energy that may escape past the conductive gasket  14  must traverse the first wall  28 . However, in order to get past this stage, the energy will face the inner groove  52  with absorbent material  62   c . Whatever energy that may escape must traverse the inner door flange  54 . In order to get past this stage, the energy will face the first door seal groove  38  with absorbent material  62   a . Whatever energy that may escape must traverse the second cavity wall  32 . In order to get past this stage, the energy will face the outer groove  58  with absorbent material  62   d . Whatever energy that may escape must traverse around the outer door flange  56 . In order to get past this stage, the energy will face the second groove  40  with absorbent material  62   b . Each time the energy must make a turn, it faces a low angle of incidence. As used herein, this term is used to mean that the angle is close to normal. One intent of the design is to force the angle of the incident wave to be as close to normal as possible. Each time the energy must make a turn, it also contacts the absorbent material  62 . 
         [0031]    In one example, the absorbent material may be formed of silicone, a natural or synthetic rubber, or any other carrier that can serve as a binder and/or carrier. A ferrite or ferromagnetic material may be embedded within the silicone binder. Any material that has the property to absorb the leakage of energy may be used. Non-limiting examples of materials include but are not limited to alnico, bismanol, chromium oxide, carbon, cobalt, dysprosium, fernico, ferrite (iron or magnet), gadolinium, heusler alloy, iron, magnetite, metglas, MKM steel, neodymium magnet, nickel, permalloy, rare-earth magnet, samarium-cobalt magnet, sendust, suessite, yttrium iron garnet, or any combination thereof. 
         [0032]    The absorbent material may be formed as a ring-like gasket that can be wedged within each of the grooves described. The absorbent material gasket may be formed so that it does not extend the full height H of each U-shaped space formed by the grooves. This can allow each groove  38 ,  40  on the cavity seal  20  to receive a corresponding flange  54 ,  56  of the door seal  24 . This can allow each groove  52 ,  58  on the door seal  24  to receive a corresponding wall  28 ,  32  of the cavity seal  20 . 
         [0033]    As the door  22  is moved from an open position to a closed position as shown in  FIGS. 6 and 6A , it can be seen that closure of the door  22  against the cavity opening  18  causes this receiving action to take place. This configuration provides a series of convoluted bends that the energy must traverse in order to escape the microwave oven. Each absorptive material gasket  62  at each bend may reduce the emissions from about 6 dB to about 10 dB. 
         [0034]    Escaping power is forced to follow a path that causes it to meet the absorbent material at a low angle of incidence, which maximizes the effectiveness of the material. Additionally, the bends themselves provide some attenuation even without the absorbent material in place. 
         [0035]      FIG. 7  shows an alternate example with angled walls and flanges.  FIG. 8  shows the cavity seal  20 ′ and the door seal  24 ′ of this example as they are slightly opened. As shown, the cavity seal  20 ′ has a first wall  64 , a second wall  66 , and a cavity perimeter wall  68 . Cavity seal  20 ′ also has first and second grooves  70 ,  72 . In this example, the walls  64  and  66  are angled. This can allow the opening of door to be smoother, without parts of seal portions  20 ′,  24 ′ bumping one another. In this example, the grooves  70 ,  72  are also angled. This can result in a pointed groove area. 
         [0036]    Similarly, the door seal  24 ′ has a choke  12 ′, a choke wall  74 , an inner flange  76 , and an outer flange  78 . Door seal  24 ′ also has inner and outer grooves  80 ,  82 . In this example, the flanges  76  and  78  are angled. This can allow the door opening to be smoother, without parts of seal portions  20 ′,  24 ′ bumping one another. In this example, the grooves  80 ,  82  are also angled. This can result in a pointed groove area. 
         [0037]    As shown in  FIG. 8 , when the door seal  24 ′ is moved toward the cavity seal  20 ′, a flat upper face of the first wall  64  compresses against an absorbent material  84  positioned in the inner groove  80 . Absorbent material  84  is similar in properties and function to the absorbent material  62  described above, with a difference being that absorbent material  84  is shaped to fit into triangular, pointed grooves. (The absorbent material is shown in hatching in this figure; not every instance is numbered.) 
         [0038]    This example may provide an even tighter fit due to the angled features provided. Any escaping signal energy must traverse the walls, flanges, grooves, and absorbent material as outlined above. The energy strikes the features at low angles of incidence. 
         [0039]    The seals  20 ,  24  of  FIGS. 1-6  may be machined from aluminum. The seals  20 ′,  24 ′ of  FIGS. 7-8  may be cast as an entire structure, in order to provide the desired angled walls, flanges, and pointed grooves. 
         [0040]      FIG. 9  illustrates an even further example. In  FIG. 9 , the choke  12 ″ may be re-oriented sideways on the door seal  24 ″. This can be beneficial so that the choke  12 ″ does not encroach on the microwave side, but moves with the door. In this example, the cavity seal  20 ″ may have first and second walls  86 ,  88  that form a V-shape  90  therebetween. An absorbent gasket material  62  may be positioned therein. The first wall  86  may also support a conductive gasket  14 . This conductive gasket  14 , however, may be moved to the door seal. 
         [0041]    The door seal  24 ″ may have a flange  92  with angled side walls, such that the flange  92  is received within the V-shape  90 . The door seal  24 ″ may also have an absorbent gasket material  62  positioned such that it is compressed against second wall  88  upon closure of the door seal  24 ″ against the cavity seal  20 ″. Again, any escaping energy will be required to traverse the convoluted sequence of bends. Each bend helps reduce unwanted emissions. Each instance of an absorbent gasket material  62  helps reduce unwanted emissions.  FIG. 10  shows the door seal  24 ″ closed against the cavity seal  20 ″.  FIG. 11  shows a top view of a microwave cavity  18  with the seal configurations of  FIGS. 9 and 10 .  FIG. 12  shows a view of the seal configurations in place. The gradual taper of the mating surface (the first wall  86 ) for the conductive gasket  14  can promote a low closure force. 
         [0042]    In some aspects, the microwave seal may be provided according to one or more of the following examples. 
       Example 1 
       [0043]    A microwave oven door seal, comprising: a cavity seal and a door seal that cooperate with one another; an absorbent material stage comprising (a) the cavity seal comprising a first groove and a second groove, each of the first and second grooves comprising an absorbent material contained therein and (b) the door seal comprising inner groove and an outer groove, each of the inner grooves and outer grooves comprising an absorbent material contained therein. 
       Example 2 
       [0044]    A microwave oven door seal for an aircraft microwave, comprising: a cavity seal and a door seal that cooperate with one another; a choke seal; an conductive gasket seal; an absorbent material stage seal comprising (a) the cavity seal comprising a first groove and a second groove, each of the first and second grooves comprising an absorbent material of silicone and ferrite contained therein and (b) the door seal comprising inner groove and an outer groove, each of the inner grooves and outer grooves comprising an absorbent material of silicone and ferrite contained therein. 
       Example 3 
       [0045]    A microwave oven door seal, comprising: a cavity seal and a door seal that cooperate with one another; an absorbent material stage wherein the cavity seal and the door seal form a convoluted series of bends that force any escaping microwave energy to contact the bends at a low angle of incidence; wherein each of the bends comprises an absorbent material associated therewith. 
         [0046]    Changes and modifications, additions and deletions may be made to the structures and methods recited above and shown in the drawings without departing from the scope or spirit of the invention and the following claims.

Technology Category: h