Patent Publication Number: US-8992715-B2

Title: System and method for the rapid, automated creation of advanced composite tailored blanks

Description:
This application is a continuation of U.S. application Ser. No. 13/435,006, filed Mar. 30, 2012, now U.S. Pat. No. 8,414,729, which is a continuation of U.S. application Ser. No. 13/251,360, filed Oct. 3, 2011, now U.S. Pat. No. 8,168,029, which is a division of U.S. application Ser. No. 12/237,077, filed Sep. 24, 2008, now U.S. Pat. No. 8,048,253, which claims the benefit of U.S. Provisional Application No. 60/975,464, filed Sep. 26, 2007, all of which are herein incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates generally to advanced composites and, more particularly, to a system and method for rapidly fabricating advanced-composite laminates with low scrap using automated equipment. 
     2. Background of the Invention 
     Advanced composite materials are increasingly used in high-performance structural products that require low weight and high strength and/or stiffness. Composite materials are engineered materials that comprise two or more components. This patent relates to polymer composites that combine reinforcing fibers such as carbon fiber, glass fiber, or other reinforcing fibers with a thermosetting or thermoplastic polymer resin such as epoxy, nylon, polyester, polypropylene, or other resins. The fibers typically provide the stiffness and strength in the direction of the fiber length, and the resin provides shape and toughness and transfers load between and among the fibers. Structural performance of an advanced composite part increases with increased fiber-to-resin ratio (also called fiber volume fraction), increased fiber length, degree of fiber orientation to line up with a part&#39;s loads (in contrast to random fiber orientation), and the straightness of the fibers. Weight of an advanced-composite part can also be improved by selectively adding or subtracting material according to where it is highly and lightly stressed. 
     Typically, the manufacture of high-performance, advanced-composite parts is a slow and labor-intensive process. Thus, several approaches for automating the fabrication of advanced composite parts have been developed to reduce touch labor, decrease cycle time, and improve part quality and repeatability. Such machines are used to fabricate small and large parts ranging from entire plane fuselages to pressure vessels, pipes, blades for wind turbines, and wing skins. These machines typically place tape material directly on a mandrel or a mold using a material placement head mounted on a multi-axis numerically controlled machine. As the material is laid up, it is consolidated with any underlying layers. This is called “in situ” consolidation. 
     A different approach, described in U.S. Pat. Nos. 6,607,626 and 6,939,423, which are herein incorporated by reference, is to lay up a flat “tailored blank” where all the plies of the composite laminate are only tacked together. Once the tailored blank has been made, subsequent processing steps are used to consolidate the plies together and form the blank into its final shape. 
     One characteristic typical to tape and fiber placement machines is the overall technique used to apply material to a tooling surface: machines progressively unroll a tape or tow material onto a tooling surface and tack it in place using one or more compaction rollers or shoes as the material is fed onto the surface. While this technique yields high quality parts, one main limitation is that increasing material placement speed requires larger, higher power machines, which have a compounding effect on system size, energy consumption, cost, and precision. In many systems, speed is also constrained by heat transfer, especially for thermoplastic composites that are typically melted and then refrozen during placement. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a system and method for applying composite tape material onto a tooling surface. Instead of progressively applying tape to a tooling surface, the present invention feeds tape material of a predetermined length and locates it above the tooling surface. The tape can be pre-cut into a tape section or can be fed from a tape supply and cut into a tape section (or course) of the desired length. The tape section is moved toward the tooling surface. Once near or on the tooling surface, the entire tape section can be tacked to the underlying material in one operation. The tacking can comprise, for example, the fusing of the tape section to the underlying material at isolated locations using a spot welder. Separating the tape feed operation from the tacking operation allows the system to run automatically, faster, and more reliably. 
     An embodiment of the present invention comprises a system and method for making high-performance, advanced-composite laminates by positioning one or more tape sections over a multi-axis motion table, then lowering the tape sections onto the table and tacking them to previously laid material using an array of spot welding units. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating an exemplary tailored blank, according to an embodiment of the present invention. 
         FIG. 2  is a schematic diagram illustrating an isometric view of an exemplary system for applying composite tape, according to an embodiment of the present invention. 
         FIG. 3  is a schematic diagram illustrating a front view of the system shown in  FIG. 2 . 
         FIG. 4  is a schematic diagram illustrating a top view of the system shown in  FIG. 2 . 
         FIG. 5  is a schematic diagram illustrating a detail view of an exemplary material dispensing system and tape transport rail, according to an embodiment of the present invention. 
         FIG. 6  is a schematic diagram illustrating a detail view of an exemplary material dispensing system and autofeed unit, according to an embodiment of the present invention. 
         FIG. 7  is a schematic diagram illustrating an exemplary placement head, according to an embodiment of the present invention. 
         FIG. 8  is a schematic diagram illustrating a side view of an exemplary feed and cutter unit with the front frame plate removed, according to an embodiment of the present invention. 
         FIGS. 9   a  and  9   b  are schematic diagrams illustrating a detail view of an exemplary tape placement head, according to an embodiment of the present invention. 
         FIG. 10  is a schematic diagram illustrating a detail view of an exemplary tape carriage, according to an embodiment of the present invention. 
         FIG. 11  is a schematic diagram illustrating an end view of an exemplary carriage frame, according to an embodiment of the present invention. 
         FIG. 12  is a schematic diagram illustrating slotted holes in an exemplary carriage frame, according to an embodiment of the present invention. 
         FIG. 13  is a schematic diagram illustrating a side view of tape passing through the exit of the feed and cutter unit and entering the guide rails of the carriage frame, according to an embodiment of the present invention. 
         FIG. 14  is a schematic diagram illustrating a front view of an exemplary welder frame unit, according to an embodiment of the present invention. 
         FIG. 15  is a schematic diagram illustrating a front view of an exemplary single welder unit with a pressure foot attached, according to an embodiment of the present invention. 
         FIG. 16  is a schematic diagram illustrating an exemplary configuration of an unwind and slack system, according to an embodiment of the present invention. 
         FIG. 17  is a schematic diagram illustrating a detail view of a single autofeeder unit, according to an embodiment of the present invention. 
         FIG. 18  is a schematic diagram illustrating an end view of a tape section cupping between guide rails and a lowered guide rail vertical location relative to a tooling surface, according to an embodiment of the present invention. 
         FIG. 19  is a schematic diagram illustrating an end view of a tape section being tacked to an underlying tape section, according to an embodiment of the present invention. 
         FIG. 20  is a schematic diagram illustrating an end view of a tape section being removed from guide rails as the guide rails retract and the pressure foot holds the tape section in place, according to an embodiment of the present invention. 
         FIG. 21  is a schematic diagram illustrating an end view of guide rails retracted and pressure foot retracted, according to an embodiment of the present invention. 
         FIG. 22  is a schematic diagram illustrating lateral movement of guide rails with an angled cutter, according to an embodiment of the present invention. 
         FIG. 23  is a schematic diagram illustrating an exemplary angled cutter and horizontally actuating guide rail mechanism with the cut angle in the +45° cut position, according to an embodiment of the present invention. 
         FIG. 24  is a schematic diagram illustrating a top view of the angled cutter and horizontally actuating guide rail mechanism of  FIG. 23 , with the cut angle in the +45° cut position. 
         FIG. 25  is a schematic diagram illustrating a top view of the angled cutter and horizontally actuating guide rail mechanism of  FIG. 23 , with the cut angle in the 0° cut position. 
         FIG. 26  is a schematic diagram illustrating a top view of the angled cutter and horizontally actuating guide rail mechanism of  FIG. 23 , with the cut angle in the −45° cut position. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the present invention provides a tape placement method. As described in detail below, the tape placement method involves locating a material placement head above a tooling surface, feeding tape into a guide that holds the tape above the tooling surface, cutting the tape from the feedstock into a tape section, moving the tape section near or onto the tooling surface, and then tacking the tape section to any underlying tape layers or securing the tape section to the tooling surface. 
     Material 
     The present invention can be used to create advanced-composite preforms from tape material, such as preimpregnated fiber and polymer tape and pure polymer tape. Advanced composite materials contain reinforcing fibers, a polymer resin, and sometimes a core material such as foam or honeycomb. The part is made by stacking layers of material in one or more orientations, or ply angles. The ply angles are determined by the part&#39;s load requirements. Typically, the material that comprises the laminate will be a preimpregnated tape containing reinforcing fiber and a polymer resin. However, unreinforced polymer tape, core material, or other materials such as a metallic film could be included in the laminate based on the requirements of the application. 
     The reinforcing fibers can be made from, for example, carbon, glass, basalt, aramid, polyethylene, or any other reinforcement. Like string, rope, hair, or other fibers, reinforcing fibers used in polymer composites add strength and stiffness to a part along the direction of the fiber length. The fibers are preferably continuous or near continuous in length and aligned parallel with the tape length (or longitudinal axis of the tape), but no specific reinforcement format within the tape is required for the invention to be practiced. The polymer resin can be thermoplastic (for example, polyamide, polyphenylene sulfide) or thermosetting (for example, epoxy, vinyl ester, or polyester). As with most polymer composite applications, the specific resin and fibers used and their proportion and format within the composite are determined by the specific requirements of the part to be manufactured. 
     Process 
     The present invention can be used to create a laminate comprising multiple plies of tape material. Each ply contains one or more sections of tape (also called courses) placed parallel to each other, and each ply is fused to one or more underlying plies. The shape of each ply and the orientation, or angle, of the fibers in the ply relative to fibers in other plies in the laminate are chosen such that the final produced part will have the desired structural characteristics. The first step of the method used to fabricate a tailored blank is to feed tape material of a specified length from a spool into a tape section positioning system that suspends the tape above the tooling surface upon which the tape will be placed. The second step, which could occur concurrently with step one, involves positioning the tape section relative to the tool such that when the tape section is lowered onto the tool, it is in the correct position and orientation for the part. In the third step, the guide lowers the tape section and the section is either tacked to underlying material or secured to the tooling surface. Any material that directly touches the tooling surface can be secured to the tool using techniques (e.g., vacuum) that may differ from the methods used to tack tape layers to each other. Once the section of material has been secured to underlying material or to the tooling surface, the tape section locating system and tacking system return to their starting positions and a new section of material is fed into the guide and the process is ready to be repeated until the blank is complete. 
     Several variations or embodiments of this overall processing method could be employed. Though the process is described here as a series of sequential steps, embodiments of this method could execute these steps simultaneously to reduce overall processing time. For example, moving the tape section locating system and/or the tool into position could occur while the guide is raising and/or lowering and/or while the tape is being fed into the guide. Also, embodiments of this method could place more than one section at a time by incorporating additional placement heads into the machine. Thus, nothing in this description should imply that sections necessarily be placed one at a time. Specific embodiments of the method could include the capability to place varying widths or multiple materials in a single blank. 
     In addition, alternate embodiments of this invention could involve placing tape sections that have been pre-cut rather than cutting the tape after it has been fed into the guide rails. Staging pre-cut tape before the guide rails could speed up the operation of the machine since the cut operation would be removed from the critical path in the order of machine operation. The tape sections could be pre-cut in line with the feed and placement, or the tape sections could be pre-cut off-line and staged as a kit to be fed into the guide rails. 
     The method used to tack plies together and the degree to which they are tacked is another parameter that could vary in different embodiments. Methods for tacking the courses to underlying plies could include contact heating, ultrasonic welding, induction welding, laser heating, hot gasses, or other methods of adhering plies to each other. Also, the method could be used with an articulating head or a moving tooling surface, or a combination of the two positioning approaches. Although an embodiment described herein uses a fixed material placement head that is positioned over a flat tooling surface that can move in the x and y directions as well as rotate, the relative motion between the placement head and the tooling surface could also be achieved by moving the placement head or a combination of the two. 
     Implementation of the Invention 
     A specific implementation of the present invention is depicted in  FIGS. 1-16 . This exemplary system comprises a material dispensing system  1 , tape transport rails  2 , a placement head  3 , a vacuum table  4  that serves as the blank tooling surface, a 3-axis motion table  5 , and a structural beam  6 . The placement head  3  and tape transport rails  2  are affixed to the structural beam and positioned over the motion table  5  and vacuum table  4 . Additional support systems are depicted and include an operator interface computer  7 , an electronics control cabinets  8 , a vacuum pump  9 , and perimeter safety guarding  10 . 
     As shown, for example, in  FIGS. 5 ,  6 ,  16 , and  17 , the material dispensing system  1 , also called a creel rack, contains one or more creel boxes  35  that contain tape unspooler mechanisms  11 . In this system, a roll of tape  47  is loaded onto a spindle  12  and secured in place. Spool guards  13  are put in place to keep the tape from unraveling. The end of the tape is then fed around a first guide roller  48  between feed rollers  15  in a creel feed mechanism  14  and past a second guide roller  49 . Then one coil of the tape is looped around the entire spool, fed around an additional set of guide rollers  16  within the creel feed mechanism, and out to the auto feeder unit  17 . Once the tape is fed into the autofeeder unit, the autofeeder pinch rollers  18  engage and feed the tape through a feed guide  19  (see, for example,  FIG. 17 ) and across the tape transport rails and into the tape placement head  3 . Once the tape passes through feed rollers  23  in the tape placement head, these rollers  23  engage and the rollers  18  in the autofeeder disengage. 
     As shown in  FIG. 7 , the tape placement head  3  has three subsystems: a tape feed and cutter unit  20 , a welding unit  21 , and a tape carriage unit  22 . Tape enters through the tape feed and cutter unit  20  and through a set of driven feed rollers  23 . As shown, for example, in  FIGS. 8-10 , the feed rollers  23  pinch the tape and push it past an adjustable tape alignment plate  44 , through a tape-cutting unit  24 , and into guide rails  25  that are part of the tape carriage unit  22 . Tape is fed through the guide rails  25  until the distance between the tape end and the cutter blade  45  equals the desired strip length  26 . Once this point has been reached, the pinch roller stops feeding tape and the cutter unit cuts the tape. This leaves a section of tape  27  of a known length suspended between guide rails  25 . 
     In the feed and cutter unit depicted in  FIG. 8 , the cutter unit  24  comprises a blade and anvil between which the feed unit feeds the tape and whose movement is controlled such that when the blade shears past the anvil, the tape is cut into a tape section. Alternate cutter embodiments could also be used, including, for example, a rotary knife cutter, an ultrasonic knife, or a laser that could traverse a path across the width of the tape to cut it. With a traversing cutter, or similar device, the cut line could also be nonlinear, for example, curved or jagged. 
     The guide rails  25  are part of the tape carriage unit  22 . In this embodiment, the system comprises two substantially parallel guide rails  25  that are each attached to a set of carriage frames  28 . There is one guide rail and one carriage frame for each edge of the tape. Each carriage frame is affixed to the main welder frame  29  via linear slides  30  and a linear actuator  31 . As shown in  FIG. 12 , slotted attachment holes  42  in the horizontal portion  43  of the carriage frame allow the distance of the guides from centerline of the tape  48  to be adjusted. 
     In this embodiment, the tape section is held between the guide rails by grooves in the guide rails in which the edges of the tape rest. Tape that has a certain degree of transverse stiffness (i.e., stiffness across the width of the tape section) stays in place just by resting on the grooves. For tape with insufficient transverse stiffness to stay in the grooves, the guide rails can be moved closer together such that the tape is squeezed and becomes cupped with the center of the tape higher than its edges. The cupped shape provides additional transverse stiffness to the tape section, allowing it to be held by the guide. 
       FIGS. 18-21  illustrate an exemplary system and method for handling, placing, and tacking a tape section  56  to an underlying tape section. In this example, the tape section  56  has insufficient transverse stiffness to remain in the grooves simply by resting on top of the lower surface of the grooves in the guide rails  25 . Therefore, the guide rails  25  are close together such that edges of the tape section  56  are squeezed by the vertical walls of the grooves, causing the tape section  56  to become cupped with the center of the tape section higher than its edges  53 , as illustrated in  FIG. 18 . In other words, the width of the tape section  56  is convex with respect to horizontal in  FIG. 18 . The cupped shape provides additional transverse stiffness to the tape section, allowing it to be held by the guide rails  25 . 
     With the tape section  56  within the guide rails  25 , the guide rails  25  lower to a point just above the tooling surface  4 , as illustrated in  FIG. 18 . Next, as illustrated in  FIG. 19 , one or more pressure feet  54  lower to press the tape section  56  to the underlying material. In the present embodiment of the invention, the welder tips  46  themselves serve as pressure feet in addition to the dedicated pressure feet  33  that have also been described. Alternatively, the welder tips and pressure feet can be a single integrated device. With the tape in place and the welder tips lowered, the tape section is then tack welded to any underlying material. In some cases, the tape section will directly contact the tooling surface and in other cases it will contact previously placed tape sections  55 . If the tape section underneath a welder tip is directly touching the tooling surface, then that welder does not energize, since that portion of the tape section is secured to the table, for example, with vacuum. 
     Once the welds have been made, as illustrated in  FIG. 20 , the guide rails  25  rise up with the pressure feet still pressing on the tape. This causes the tape edges to slip out of the grooves of the guide rails  25 , releasing the tape section  56  from the guide rails  25 . Then, as illustrated in  FIG. 21 , the pressure feet retract, leaving the tape section  56  secured in place on the tooling surface  4 . 
     The vacuum table  4  serves as the tooling surface upon which the part is built up. The vacuum table is attached to a three-axis motion platform  5  that precisely positions the vacuum table under the placement head. Instructions from an off-line program determine the length of tape and its position and orientation on the table. While the tape is being fed into the guide rails  25 , the motion platform  5  moves the vacuum table under the placement head to the desired x-axis, y-axis, and rotary angle coordinates. 
     When the tape has been fed into the guide rails  25  and cut from the roll into a section, and the vacuum table  4  has been positioned in the desired location, the carriage frames  28  lower until the tape section is nearly touching a previously laid ply or the tooling surface. Then an array of one or more welder units  32 —six welder units are illustrated in  FIGS. 2 ,  4 , and  7 —lowers from the welding unit  21  and presses the section of tape onto the tool. In addition to the welder tip  46 , a pressure foot  33  is attached to each welder actuator (see, for example,  FIGS. 14 and 15 ). This pressure foot also presses down on the tape to hold it in place until it has been tacked to the underlying material. 
     At this point, the tape under each welder tip is either directly contacting the vacuum table  4 , which is the tooling surface, or is resting on top of previously placed material. In the first condition, the tape section is held in place by the table&#39;s vacuum. In the second case, the welder will turn on and tack the top layer of tape to the underlying material. In an embodiment of the invention, the welders use an ultrasonic impulse to create a weld. 
     When the welding/tacking step is complete, the welder units remain in contact with the tape and the carriage frame begins to lift up and return to its load position. Since the center of the tape remains secured to the tool and pinned down by the welder units, this causes the edges of the tape to slide out of the guide rails  25 . Once the tape edges are free of the guide rails  25 , the welder units retract, leaving the tape section in place on the vacuum table tooling surface. 
     This procedure is repeated for each section of material that will comprise the blank. After the final tape section has been placed, the table vacuum is turned off, which allows the part to be removed easily from the table. 
     For the placement head  3  to feed tape quickly into the guide rails  25 , the tape preferably has very low back tension. Without active tension control, the inertia of the spool of material could resist the rapid acceleration and deceleration of the tape as the feed rollers push tape into position. Thus, as shown in  FIG. 16 , the material dispensing system  1  actively unspools the material roll so that it puts slack in the final loop of material  34  within the creel box  35 . The amount of slack is determined by the length of the tape section to be placed. This slack loop leaves very little back tension in the tape and allows the placement head to feed material very quickly. Sensors within the creel box  36  inform the control system if the slack loop is getting too small or too large. 
     The pinch roller can feed tape from a spool into a slack coil of tape within a creel box and can be configured to maintain the slack loop size to within a determined range using feedback by one or more slack loop position sensors. Maintaining slack at the end of a tape spool reduces back tension in the tape as the tape feeds. Actively uncoiling the tape spools of the tape supply unit minimizes tape tension between the tape feed unit and the tape spools. 
     During operation, several events can interrupt the normal sequence of actions. Several sensors are put in place to flag such events and to handle them. One event is the feed roller failing to feed the correct amount of tape. This has been addressed by incorporating an encoder wheel  37  ( FIGS. 9   a  and  9   b ) that measures how much tape is being fed. If the measured feed length does not agree within a certain tolerance to the desired feed length, then corrective action is taken. 
     The system also senses when the end of the roll of material has been reached. When the end of the roll is reached, a sensor  38  ( FIG. 17 ) in the autofeeder unit  17  detects the end of the tape. The system continues to feed the tape until a second material detection sensor  39  ( FIG. 8 ) at the entrance to the placement head  3  detects the tape end. When the tape end passes this point, the feed roller stops and a redirect plate  40  within the placement head is engaged, pushing the tape end downward. The motion platform then moves the vacuum table out from under the placement head and the feed roller  23  reverses its motion and thereby feeds the final strip of tape into a catch basin  41 . Then, if the second creel box is loaded with the same tape, the system can run the autofeeder to load the placement head with new material and operation can continue without further delay. 
     The second creel box can be used for more than one purpose. If the tailored blank includes more than one type of tape material (for example, glass fiber tape and carbon fiber tape), then the creel boxes could be loaded with different materials. Then, the placement head would be able to lay up two types of material without having to be reloaded manually. Rather, the pinch rollers in the autofeed unit of one creel box could retract tape a sufficient distance so that tape from the other creel box can be fed into the placement head. 
     In the embodiment described herein, the machine can be set up to process different tape widths. To change widths of tape to process, adjustment is made to the creel boxes so that the centerline of the tape roll is aligned with the centerline of the welder tips. The distances between the guide edges of the feed guides, tape transport rails, and carriage frame are then widened or narrowed to accommodate the desired tape width. In one embodiment, the adjustments are made manually. Another embodiment employs automatic width adjustment. This automatic adjustment can speed the setting up of the equipment to run different widths of material and it can allow multiple tape widths to be laid up in a single blank without having to stop and adjust the equipment. 
     An alternative embodiment of the present invention adds the ability to cut tape so that the cut edge is not always perpendicular to the sides of the tape. Adding the ability to vary the cut angle can give tailored blanks more degrees of freedom that can in turn lead to improved blank performance. An exemplary angled cutting system is illustrated in  FIGS. 22-26 , according to an embodiment of the present invention. In this system, cutter unit  50  rotates about its center and guide rails  51  each move forward and backward to maintain a close distance between the cut edge  52  of the tape section  26  and the guide rails  51 . In the embodiment depicted, the horizontal movement is achieved by adding horizontally oriented linear slides  57  and actuators to the welder frame  59  upon which the vertically oriented linear slides  58  and actuators are mounted. When the cutter unit rotates to a specified angle (the system configuration at three different angles are depicted in  FIGS. 24-26 ), one side of the cutter moves toward the guide rails and the other side moves away from the guide rails. This movement can be matched by the guide rails to maintain a minimum gap between the cutter and the rails. Maintaining a minimal gap ensures that only a small portion of the tape section overhangs the guide rails, and prevents the end of the tape section from sagging off the end of the guide rails and hindering the handling of the tape section. 
     Another variant of the system described includes a carriage system that moves the guide rails not only vertically, but also horizontally along the tape feed direction. This movement allows more precise positioning of the tape relative to the welder heads, and it allows for courses to be placed that are shorter than the distance between the cutter blade and the first welder head. After the cutter cuts a tape section, and the tape is within the guide rails, the carriage moves horizontally away from the cutter and lowers toward the tooling surface to deposit the tape section. Lateral guide rail movement is thus included as an independent concept, though it may also be a feature of the angled cutting system. 
     In an embodiment of the present invention, either or both of the tooling surface and guide rails move to provide relative displacement with respect to each other. Such movement can be in any direction, such us horizontal, vertical, or combinations thereof, and can include movements such as translational, rotational, or pivotal movements. 
     As one of ordinary skill in the art would appreciate, the guide rails described herein are only one of many means that could be used for suspending the tape above the tooling surface and positioning the tape once it has been fed and cut. Other methods that use vacuum to suspend the tape above the tooling surface, trays that release the tape by increasing the distance between the guide rails, retractable trays, or other mechanisms may be employed. 
     The foregoing disclosure of the preferred embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims appended hereto, and by their equivalents. 
     Further, in describing representative embodiments of the present invention, the specification may have presented the method and/or process of the present invention as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present invention.