Abstract:
A mechanism for constraining a product package as it is conveyed through a microwave applicator. In one preferred embodiment, the conveyor is provided by a pair of parallel guide belts arranged to travel through a microwave applicator. The guide belts have a series of slots or other guide elements formed therein. The guide elements engage edges of the product packages. In one arrangement, the pair of belts are disposed on either side of a series of packages, and thus constrain the location of the packages in three dimensions.

Description:
RELATED APPLICATION(S) 
     This application claims the benefit of U.S. Provisional Application No. 61/192,687, filed on Sep. 19, 2008. The entire teachings of the above application(s) are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a conveyance system for use in microwave processing of products such as, for example, the sterilization and/or cooking of food articles, and in particular to a constraint mechanism for holding conveyed items in place providing minimal mechanical interference with the microwave energy applied to improve the uniformity of results in such systems. 
     The processing of products with microwave energy, such as needed for cooking, sterilization, and similar operations, involves placing the product within some sort of enclosed waveguide applicator for a time. Many different approaches have been tried to improve the uniformity of results in such systems. These usually involve carefully designing the shape of the waveguide applicator. 
     For large scale, high volume, commercial applications, it is typically desirable to process as many items in as short a time as possible. In these situations, it is common to use a continuous conveyor to transport items through the applicator. Conveyor belts made of materials that are suitable for use in such environments are well known. For example, materials such as Teflon™, Kevlar™ or fiberglass have been used for conveyors in microwave ovens. 
     In addition to ensuring that the microwave applicator is of proper dimension, it is also important to ensure that the position of the product with respect to the waveguide is carefully controlled. There is no advantage to having a precisely dimensioned microwave applicator if consistent positioning of the product within the applicator cannot be assured as it travels through the applicator. If the product moves up and down or from side to side while being conveyed through the applicator, uneven results will occur. 
     One manner of constraining conveyed packaged items is described in a co-pending U.S. patent application Ser. No. 11/335,837, filed Jan. 19, 2006 (U.S. Patent Publication No. 2006/0231150) entitled “Product Guidance System for Continuous Conveyor Microwave Oven”, which is assigned to The Ferrite Company, Inc. The approach in that system was to use a series of fixed wire guides located adjacent the conveyor. 
     Many different approaches have been attempted, all of which utilize a mechanical structure that spans the microwave exposure area with some form of traditional conveyor belt or other transport mechanisms. These methods introduce significant mechanical structures into the microwave heating area (ie. Kevlar mesh belts, plastic link belts or other plastic cradle structures) causing interference with the microwave field and loss of uniformity in heating. The benefits of a carefully designed waveguide applicators are diminished by undesirable interaction with the microwave energy created by the mechanical structures. 
     SUMMARY OF THE INVENTION 
     The present invention relates to mechanisms for constraining a product package and conveying it through a microwave applicator. In one preferred embodiment herein, the conveyor is provided by a pair of parallel guide belts utilizing the structure of the conveyed product. The guide belts are arranged to travel through a microwave applicator. The guide belts have a series of slots or other guide elements formed therein that engage the edges of the product packages. By disposing the pair of belts, one on either side of a series of packages, the packages are supported and the location of the packages is constrained in three dimensions as they pass through a microwave applicator. Advantageously, no additional mechanical support is required for engaging and supporting the packages in the vertical direction or for transporting the packages horizontally through a liquid or gaseous environment within a microwave processing chamber. 
     Tensioning is preferably provided to both hold the belts in place and to provide a drive mechanism to create the conveyor. This can be provided by a series of pulleys driven by a motor. 
     In a preferred arrangement, the portions of the drive mechanisms located within the applicator section is ideally limited to including essentially the belt portions. This avoids introducing undesirable interaction with the microwave energy applicator. The belts themselves are thus preferably made of a suitable material that is temperature, fluid and microwave energy insensitive. Teflon™, Kevlar™ and fiberglass are examples of suitable materials. Polyprophelene and polyethylene are probably not as suitable as other materials. 
     The pulley may have sprockets that engage holes formed in the drive belts. This provides further positive drive engagement, and avoids belt slippage under weight of the conveyed packages. 
     In one embodiment, the packages can have specially shaped tabs on the outer periphery. The tabs are shaped to better engage the slots in the belts, reduce variation in package positioning and reduce tension requirements on the belts. 
     In optional arrangements non-rigid packages are conveyed utilizing tabs or slots on the periphery. These are engaged by an alternate belt configuration that grasps the integrated features for tensioned conveyance between the belts. 
     In optional arrangements, tension measurement devices and other devices such as motion transducers may be used in combination with the belt to provide feedback on package location and condition. The tension measurement devices can be located preferably outside the active area of the applicator but can also be provided within it. 
     There are several benefits provided by a conveyed product guidance system according to the present invention. 
     Elimination of significant mechanical structures for supporting and transporting the packages in the microwave heating area reduces interference with the applied microwave energy and resulting in improved uniformity in microwave heating. 
     The constraint mechanism maintains the location of product packages in three dimensions without additional mechanical support while they travel through a microwave applicator. This improves the uniformity of microwave processing of packaged articles. 
     In environments where products must also travel through water, vegetable oil or other fluids while being processed such as is useful for example, pasteurizing the same, turbulence is introduced or increased by the movement of the conveyor belts, thereby improving mixing and heat transfer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
         FIG. 1  is an isometric view of a package conveyor. 
         FIG. 2  is a more detailed view of a pair of conveyor belts and how they engage the product packages. 
         FIG. 3  is an exploded view of the conveyor belts and packages. 
         FIG. 4  is a top view of the package conveyor. 
         FIG. 5  is a side view of the package conveyor. 
         FIG. 6  is an isometric view of a package conveyor using supports. 
         FIG. 7  is a more detailed view showing how a notch in a support relates to an edge of the packages. 
         FIG. 8  is a detailed cross sectional view of a support and package. 
         FIG. 9  illustrates a package having tabs shaped to engage the conveyor belts. 
         FIG. 10  is another implement showing conveyor belts that use drive rollers having sprockets. 
         FIG. 11  is a close-up of the drive roller of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A description of example embodiments of the invention follows. 
       FIG. 1  is an isometric view of a product conveyor system  10 . The system  10  is disposed within a microwave applicator (not shown) which may be designed according to any of the techniques described in U.S. Pat. No. 7,002,122, entitled “Choke Assembly for Continuous Conveyor Microwave Oven” and/or U.S. Pat. No. 7,256,377, entitled “Coupled-Waveguide Microwave Applicator for Uniform Processing,”, both of which are incorporated herein by reference, and/or other known techniques for constricting continuous feed microwave applicators. 
     Conveyor system  10  is generally employed to cause items such as packaged food trays  20  (also called the “packages” herein) to be conveyed through a confined area such as a microwave cavity. Although a conveyor belt  30  portion of system  10  itself is mostly disposed within the cavity, as will be understood shortly, certain other portions, such as motor  40 , may be advantageously disposed outside of the cavity. 
     More particularly, a conveyor system  6  consists primarily of a pair of belts  30 -L and  30 -R. The conveyor belts  30  are supported above a base  32  portion of the applicator (the other walls of the applicator are not shown for clarity). Conveyor belts  30  in turn support packages  20  such that the packages  20  are spaced above the base  32 . 
     A motor  40  and series of belts, gears, and pulleys are used to drive the pair of conveyor belts  30  in synchronism. In particular, these may include a timing belt  44  and first timing belt pulley  49  coupled to motor  40  via drive gears  48  and drive shaft  50 . A second timing belt pulley  49  on the far side operates belt drive roller  43 . Belt drive roller  43  and various belt guide rollers  42  further provide movement and support for the conveyor belts  30 . 
     As illustrated, the motor  40  may be supported by motor bracket  45  on base plate  32  above gears  48 , pulley  49  and drive shaft  50 , but it should be understood that other arrangements are possible. 
     In operation, a left hand belt  30 -L is continuously fed around its respective guide rollers  42  and drive roller  43  (driven by timing belt  44 ), and a respected right hand belt  30 -R is continuously fed around its respective drive roller  43  (driven by one of the gears  48 ) and its respective guide rollers  42 . Left hand belt  30 -L and right hand belt  30 -R are thus kept in alignment with one another via gears  48 , pulleys  49  and timing belt  44 . This is necessary to ensure that the packages  20  are conveyed in an orderly fashion. 
       FIG. 2  is a more detailed view of the manner in which the trays  30  are supported by the pair of belts  30 -L and  30 -R. 
       FIG. 3  is an exploded view similar to that of  FIG. 2 . It is more evident in this view that each of the conveyor belts  30  has formed therein slots  60  spaced apart from one another. The size of the slots  60  correspond to the sizes of peripheral edges of the trays  20 . Thus a side (or edge) portion of each package  20 , as indicated by reference number  65  in  FIG. 2 , protrudes through a hole  60  on each respective left and right hand side. 
       FIG. 4  is a top view of the arrangement of packages  20 , conveyer belts  30 , guidance rollers  42 , and motor  60 , showing portions of the apparatus  10  that may be disposed within the cavity  70 . As indicated by dotted lines in  FIG. 4 , motor  40 , brackets  45 , drive pulleys  43 , etc. are preferably placed outside the cavity, only belts  30  and guidance rollers  42  are within cavity  70 . 
       FIG. 5  is a side view illustrating how conveyor  10  holds packages  20  above the bottom  32  of cavity  70 . The containers themselves may or may not be specially designed and the sizes of the slots  60  are chosen to correspond to the dimensions of the edges of trays  20 . 
     It is furthermore advantageous to have as thin a cross-section belt  30  as possible, such that it will not alter the electromagnetic fields within the applicator. Materials for the conveyor should also be chosen to be relatively strong, as a relatively long length conveyor may be needed. Teflon™, Kevlar™, fiberglass and other microwave-inert materials are suitable for belts  30  and guidance rollers  42   
     A further advantage is provided by the individual slots  60 . Even if there is a change in the volume of one package, i.e., a sealed package is breached for some reason during cooking, the disturbance of a given package will not necessarily effect orderly conveyance of its neighbor packages through the applicator. 
     Therefore, in an optional arrangement, a tension measurement device or other device such as a motion transducer may be used in combination with the belts  30  to provide feedback on package location and condition. The tension measurement devices can be located as part of one of the guidance rollers  42  within the cavity  70  or may be part of one of the drive pulleys  42  located outside the active area of the applicator. 
     For some end uses, the size of the belts  30  may need to be somewhat long. This can be to accommodate larger sized packages  20 , a larger microwave applicator, or for other reasons. In such instances, the belts alone  30  may not adequate to support the weight of the trays  20 , in which case the belts may sag. 
     To prevent this for happening, conveyor supports may be added. One preferred implementation, shown in  FIG. 6 , has two supports  80 -L- 1 ,  80 -L- 2  holding up the left side and two supports  80 -R- 1  and  80 -R- 2  holding up the right side. Note that in this preferred arrangement, supports  80  have longitudinal notches  81  that are intended to engage the side edges  65  of packages  20  and not the belts  30  themselves. 
       FIG. 7  shows the notch and package edge  65  in more detail. 
       FIG. 8  is a cross sectional view of the support  80  and package  20 . 
     In certain embodiments, the packages  20  may have edges  65  that have a shape specifically adapted to engage the slots  60 . 
     In the example shown in  FIG. 9 , the package  20  includes front and rear edges  66  and side edges  65 . The front and rear edges  66  preferably intersect the side edges  65  at rounded corners  69 . Each of two side edges  65  are formed with tabs  67  extend laterally beyond the rounded corners and preferably beyond an upper lip  68  of each side edge  65 . The width and height of tabs  67  are chosen so as to firmly engage and be received within the belt&#39;s slots  60 . It should be understood that other shapes for tabs  67  and slots  20  are possible. For example, each side of the package  20  may have multiple tabs  67 . 
     The number and location of the supports  80  can differ depending upon the length of the conveyor and other factors, such as the weight of the packages  20 , or the size of the applicator cavity. 
     The supports  80  are preferably located outside of the applicator, to avoid impacting the desired radiation pattern. This is especially important in a case where the conveyor moves the packages  20  through water, such as may be done in a sterilization operation. 
       FIG. 10  is an isometric view of an arrangement similar to that of  FIG. 1  but where a row of holes  90  are formed in the belts  30 -L and  30 -R. Best visible in the close-up view of  FIG. 11 , sprockets  92  are formed in one or more of the drive roller  43  and/or guide roller(s)  42 . The sprockets  42  engage holes  90  to provide a more positive engagement mechanism between motor  40  and belts  30 . 
     In addition, tensions  98  can be used to prevent slack in the belts  30 . However, in other end uses, it is possible to form the supports from a microwave inert material, and introduce them into the cavity. 
     While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.