Abstract:
A three-dimensional object fabrication apparatus is disclosed. A housing encloses a work area having at least two distinct zones including a fabrication zone and an assembly zone. Digital data defining geometry for a three-dimensional object to be fabricated is passed via an interface to a processor within the housing. A fabrication mechanism in the fabrication zone to fabricates sub-elements where each sub-element corresponds to a portion of the object substantially consistent with the digital data. An assembly mechanism in the assembly zone joins the sub-elements in relative alignment to form the object defined by the geometry.

Description:
FIELD 
     Embodiments of the invention relate to three-dimensional object fabrication techniques. More specifically, embodiments of the invention relate to rapid fabrication of arbitrary three-dimensional objects. 
     BACKGROUND 
     The state of the art in fabrication of arbitrary three-dimensional objects is fused deposition modeling (FDM) in which tiny deposits of plastic analogous to a pixel in a three-dimensional model are deposited individually to build a desired object from the ground up. Among the problems facing FDM are speed and cost. Because each subsequent deposit fuses to the underlying previously deposited plastic, the size of the deposit and the temperature control required to effect the fusing is strictly limited. As a result, very small amounts of plastic are deposited with each deposition and if the temperature is not precisely controlled, failure along corresponding knit line is manifestly likely. Moreover, because of the small amount of each deposit, the other constraints of controlling the system during fabrication, the time required to produce even a relatively simple object is measured in hours. 
     Among the additional problems includes the need to insure desiccation of the plastic supply as moisture in the supply further causes the risk of failure of proper knit during fusing. Also, because of the small amount of plastic deposited any overhang cantilevered portion of the object must be supported by a sacrificial material that is laid down during the fabrication process and then dissolved away post-fabrication. The sacrificial material requirement increases the cost and time required to fabricate any particular object. Typically, both the sacrificial material and the build plastic are provided as a spool often costing hundreds of dollars for a relatively small volume of plastic. Moreover, if there is insufficient plastic remaining on the spool to complete a desired build, the spool must be removed and replaced and it is difficult to change spools mid-process or reuse a partially consumed spool. This further increases the cost associated with FDM. 
     A faster lower cost system with higher reliability is desirable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments of the invention are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” embodiment of the invention in this disclosure are not necessarily to the same embodiment, and they mean at least one. 
         FIG. 1  is a diagram of an external view of a fabricator of one embodiment of the invention. 
         FIG. 2  is an internal view of the fabricator of one embodiment of the invention. 
         FIG. 3  is a center vertical sectional view of a fabricator of one embodiment of the invention. 
         FIG. 4  is a view of the melt impeller of one embodiment of the invention. 
         FIG. 5  is a cross-sectional view in which the build surface and extrusion nozzle can be seen. 
         FIGS. 6A-C  are internal views showing the addition of a sub-element to a partially constructed object. 
         FIG. 7  is a schematic representation of the welding process of one embodiment of the invention. 
         FIG. 8  is an internal view of one embodiment of the invention with a mill arm extended. 
         FIG. 9  is a schematic diagram of one embodiment of the invention. 
         FIG. 10  is a schematic diagram of an embodiment of the invention permitting modular expansion. 
         FIG. 11  is a flow diagram of operation of one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a diagram of an external view of a fabricator of one embodiment of the invention. Housing  110  has a portal  114  in its top surface  118 . Portal  114  may be accessed through sliding door  116  to load a material supply into the fabricator and also to retrieve a fabricated object there from. In some alternate embodiments, separate portals are provided for loading the material supply and retrieving the fabricated object. In some embodiments, the portal  114  represents less than 50% of the area of the top surface. Interface unit  112  may provide USB ports, wireless antenna, an Ethernet port, or other suitable media ports that facilitate the passage into the unit of three-dimensional data representing at least a portion of the geometry of a substantially arbitrary object to be fabricated. 
       FIG. 2  is an internal view of the fabricator of one embodiment of the invention. Internally, there is an assembly zone  200  and a fabrication zone  202 . Sub-elements of the ultimate three-dimensional object to be created are fabricated within fabrication zone  202  and then put together (assembled) in assembly zone  200 . To a large degree, the maximum size of the object to be fabricated is constrained by the size of the assembly zone  200 . To a lesser degree, the size of the fabrication zone affects the ultimate size of the object. In many cases, the sub-element fabricated in fabrication zone  202  will be an entire layer of the object. It should be understood that sub-elements may be a portion of a layer or other portion of the object consistent with the geometry that is the source of the build instructions. It should be further understood that to be “consistent with the geometry”, the sub-element must have a size greater than a single voxel/pixel as a single voxel cannot be deemed to correspond to any distinct region with the geometry. 
     Within assembly zone  200  resides a build platform  234  including a turntable having a build surface  208  on which the object to be fabricated is assembled. Build surface  208  is provided with a rotational axis and a vertical axis. A driver within build platform  234  raises and lowers the build surface  208  along leadscrew shaft  232  to provide the vertical axis. An additional driver within build platform  234  rotates build surface  208  so that the object being fabricated can have any rotational orientation desired. In an alternative embodiment, the build platform may have x and y drives instead of a rotational drive, but such embodiment requires a larger assembly zone for the same size object. 
     When build surface  208  is driven to the top of its vertical range, it is exposed through the portal ( 114  of  FIG. 1 ), which permits easy access to a fabricated object. 
     In one embodiment, a material supply for fabrication is a monolithic helically-threaded plastic ingot  220 . The helical thread acts as part of the drive that advances the material supply, and may be consumed and eliminated as the material is used. In alternative embodiments, the thread may be replaced with gear teeth, and index step, a flange or series of flanges or the like, all features that may form part of the drive system to advance the material. 
     Build platform  234  is driven to its maximum height to be exposed through the portal. Any time the build platform  234  is at its maximum height, to protect the vertical axis from misalignment, one or more shear pins engage the platform so that vertical or lateral forces applied by a user are not applied to the drive servo or the vertical axis. An ingot is then loaded thereon and build platform  234  is then driven to its minimum height along shaft  232 . Plastic slide  230  then engages the plastic ingot  220  from build surface  208  and transports it laterally to be accessed by the fabrication zone  202 . Extrusion collar  224  is lowered over the ingot  220  and a peripheral helical ball screw drive therein engages the helical threading of the ingot  220  to allow the extrusion collar  224  to draw the ingot  220  upward. While other ways of lifting the material supply are possible such as a jack type lifter etc., the collar drive reduces the vertical space requirements over those alternatives. A bent leaf spring  226  automatically engages an index slot  228  that runs the length of the cylindrical ingot  220  to prevent the ingot  220  from turning while the collar drive is attempting to raise it. An internal drive  228  within the fabrication zone  202  then draws the ingot into the extrusion collar  224  so that it may provide source material for molder  204 . 
       FIG. 3  is a center vertical sectional view of a fabricator of one embodiment of the invention. A processor  360  receives geometry for an object to be fabricated via one or more of the interfaces ( 112  from  FIG. 1 ). Processor  360  then controls the drivers to control the fabrication and assembly of sub-elements described below. 
     As can be seen, fabrication zone  202  includes a molder  204  having a compressor  328  that receives plastic from ingot  220 . Extrusion collar  224  draws the ingot  220  up into contact with melt impeller  326 , which melts and shaves the plastic and routes the resulting molten plastic via compressor  328  to an extrusion nozzle  330 . Compressor  328  provides a reservoir to allow a relatively large instantaneous supply of plastic without requiring high pressure or rapid acceleration of the entire material supply. Extrusion nozzle  330  is maintained in linear relationship with the compressor and does not translate within the fabrication zone. This linear relationship and lack of translation of the nozzle and supply source allows the material supply melting to occur at lower pressure, while the smaller material reservoir in the compressor  328  can operate at high pressure and therefore draw at greater speed. However, nozzle  330  may rotate and may be narrowed or widened, as is discussed more fully below. 
     When the ingot  220  is mostly consumed, e.g., 90%, in one embodiment a new ingot can be added to follow on behind the mostly consumed ingot reducing or eliminating waste. The level of consumption required before addition of an additional ingot is, to some degree, dependent on the length of the extrusion collar as the consumed ingot should be sufficiently inside the collar such that the collar can engage the helical threading of the additional ingot. A sensor may be included to measure material supply usage and report the supply level via the processor. It is desirable to report not only a “supply low” condition, but also the volume of supply remaining so that a user can know if sufficient material exists to complete and intended build. 
     Extrusion nozzle  330  is flush with a temperature-controlled plate  340  and draws a desired sub-element on a temperature-controlled receiving plate  342 . Plates  340  and  342  are retained in parallel relation. Receiving plate  342  may be slightly textured to allow improved grip by the molded plastic. The slight texture or surface pitting allows the plastic molded thereon to grip or, in other words, sustain a greater lateral force than is sustainable by plate  340 . Plate  340  has a smooth surface to allow the molded, cooled layer or sub-element to glide over the surface and not stick thereto. For simplicity of discussion, we shall refer to sub-elements as “layers”. However, one should understand that the discussion is equally pertinent to sub-elements generally. 
     By controlling the temperature of both receiving plate  342  and plate  340 , efficient cooling of the molded material can be assured. In one embodiment, heat absorbed buy one or both plates in cooling the molded plastic is recycled and returned to the melt heater via a heat pump to improve the energy efficiency of the system. 
     The space  344  between plate  342  and  340  defines the thickness of the layer. Thus, by varying the distance between the two plates, different thickness layers of the ultimate object may be achieved. In one embodiment, receiving plate  342  is driven by a driver to control the distance between plates  340  and  342 . The desirable thickness of a layer may depend on the variability of the edges of the object being fabricated. For example, where the edge is uniformly vertical over a distance a thicker layer up to that distance may be used. But where the edge is very irregular thinner layers to accommodate that irregularity or slope may be desirable. Notably, the thickness of the layer is not tied to voxel dimension. As explained below with reference to  FIG. 8 , minor irregularities can be addressed with an integral subtractive tool existing in some embodiments of the invention. 
     Additional drivers move receiving plate  342  relative to nozzle  330  to permit the arbitrary layer to be drawn. Because the aperture of nozzle  330  is variable width and can rotate, thicker or thinner walls may be drawn. In some embodiments the angular orientation or the extrusion may be controlled by for example air jets or water jets adjacent to the nozzle  330 . In other embodiments the nozzle  330  may pitch to mold an angled wall. In still other embodiments, a mechanical roller or wiper may be use to profile the side walls of the layer before or after they have hardened. 
     As with all molders, instances will occur when the molder needs to be purged to eliminate degraded build material, etc. In one embodiment, to minimize waste and the space required for its containment, the purged material may be drawn as a disk of a desired diameter on the receiving plate  342 . A waste tube having a minimally greater diameter may be provided near the fabrication zone. The receiving plate  342  may then be driven over to align the purge disk with the tube, lower the purge disk into the tube and translate away, thereby scraping the purge disk off into the tube. Subsequent purges will stack in the tube like quarters in a roll and reduce the waste storage requirements. 
     In one embodiment, ingot  220  formed having the helical threading also has a hydrophobic coating  320  which repels moisture but is also consumed as the ingot is melted. Beneath the hydrophilic coating is a core  322  of, for example, ABS or other suitable thermoplastic which forms the primary material of fabrication. Wax-based compounds may also be used as the thermally formable material for some applications (such as lost-wax casting mandrels). Typically the core  322  will exceed 70% by volume of the ingot and more commonly will exceed 95% by volume of the ingot  220 . This provides a very high density of material supply in a single piece form factor. Such an ingot is effectively self packaged reducing waste and production costs. In some embodiments the core  322  can be formed unitarily as a whole. In other embodiments, the core  322  is formed by first molding a shell and then filling the shell with additional material. 
     Within the assembly zone, the driver  334  to drive the vertical and rotational components of build surface  208  is shown. Build surface  208  is part of a turntable that rotates on bearings  338  when driven by driver  334 . The turntable includes a plurality of part-off rings  336  which are flush with build surface  208  during assembly. Once assembly is complete, rings  336  can be driven to elevate above surface  208  to separate the fabricated object from the build surface  208 . This avoids a prior art problem that the object must be split off of a build platen by hand. 
       FIG. 4  is a view of the melt impeller of one embodiment of the invention. Melt impeller  326  resides within extrusion collar  224 . As extrusion collar  224  draws the plastic supply up into contact with melt impeller  326 , the melt impeller is heated to melt the surface of the plastic. Additionally, melt impeller  326  rotates and the channels  402  of melt impeller  326  channel the molten plastic towards the center portal  406  and into the extrusion compressor. Melt impeller has a relatively large surface area in contact with the surface of the ingot to increase the melting efficiency. The edges  404  of the channels  402  also serve to shave off small pieces of the plastic, further facilitating melting and allowing a relatively large volume of plastic to be delivered in a short time without excessive pressure. 
       FIG. 5  is a cross-sectional view in which the build surface and extrusion nozzle can be seen. In this view, build surface  208  and part-off rings  336  are visible. Additionally, the surface of temperature control plate  340  and extrusion nozzle  330  are shown. Extrusion nozzle  330 , in this embodiment, is a slot. The length and orientation of this slot may be varied to change the profile and orientation of the molded plastic. In one embodiment, the slot with is under servo control as is the rotational orientation. 
       FIGS. 6A-C  are internal views showing the addition of a sub-element to a partially constructed product. In  FIG. 6A  receiving plate  342  having layer  604  adhered thereto translates along track drive  608  into assembly area  200 . This clears a path for hot box  602  to shift downward to reside within the extrusion zone below track drive  608 . Hot box  602  is a thermally insulated storage box with one or more ohmic heaters therein. Hot box  602  retains welding iron  620  (shown in  FIGS. 6B and 6C ). Receiving plate  342  then translates back over hot box  602  along track drive  608 . The partially assembled object  606  is aligned with the layer  604  by the turntable under the control of the processor as mentioned above. 
     As shown in  FIG. 6B , receiving plate  342  then hooks or otherwise engages iron  620  and translates back into assembly zone  200  bringing iron  620  with it. Iron  620  will be maintained at a temperature to melt the surface of the plastic used in fabrication to a degree that permits adhesion between two such heated layers of plastic. To that end, a temperature sensor may be coupled thereto to ensure that the ohmic heaters in the hot box  602  are engaged when the iron  620  falls below a lower threshold temperature and disengaged when the iron  620  rises above an upper threshold temperature. The iron  620  is desirably sized to be slightly larger than the largest possible object to be assembled as this reduces the precision with which the iron must be controlled. It is also desirable that the iron  620  is selected to have a significant thermal mass as the ability to retain sufficient heat to conduct welds in fairly rapid succession reduces the wattage of the ohmic heaters required to heat the iron. Typically, iron  620  will be Teflon coated or otherwise non-stick to prevent the heated plastic from adhering thereto. It is also desirable that the material selected for the iron  620  have a high heat transfer coefficient relative to the material to be welded. A heat transfer coefficient at least twice that of the weld constituent is desirable and often it will be an order of magnitude or more higher. 
       FIG. 6C  shows the iron pressed between two elements to be connected. Once receiving plate  328  has drawn iron  620  fully from the hot box, the layer  604  is brought together with the object  606  having the iron there between. The pressure between the layer  604  and iron  620  causes surface melting on both the object  606  and the layer  604 . Typically, the entire expected contact area between the layer  604  and the expected contact surface of the object  606 . The layer  604  and object  606  are then separated to release the iron  620 . Preferably, the iron  620  is returned to the hot box  602  by, for example, stored mechanical force such as a spring. This results in a rapid return of the iron  620  and allows the surface molten layer  604  and object  606  to be pressed together creating a weld between them such that the resulting bond yields an object having substantially isotropic material properties. Because such a bond is relatively strong, it has no difficulty breaking adhesion between the receiving plate  328  and the layer  604 . Meanwhile, the hot box  602  with the iron  620  are returned to the position above the molder as shown in  FIG. 6A . 
     Layer  604  has now been added to the object  606  and a further layer may be molded and added subsequently. Because a layer has its own lateral strength, it is unnecessary to build a sacrificial layer to support it. Rather, the layers can be cantilevered or otherwise extend over a space below without an underlying supporting substrate. In the case of the first layer, its lower surface would be melted by the iron  620  and it would be adhered to the build surface. Once the object  606  is completed, the build platform  334  is driven to its full height to automatically expose the object  606  through the portal in the top of the unit. Again at this point the shear pins engage to protect the vertical access. Then, as mentioned above, elevation of the part-off rings  336  separates the object  606  from the build surface. The part-off rings also help to protect from unintentional misalignment of the vertical axis because they reduce lateral force that would be required if the user were to manually break the object off the build surface. 
     In some embodiments, once exposed through the portal, the turntable may rotate 360 degrees to provide a rotational display of the completed object. 
       FIG. 7  is a schematic representation of the welding process of one embodiment of the invention. At  702 , the two elements to be welded are aligned. At  704 , the iron is introduced between the two elements in aligned fashion. Alternatively, as previously disclosed, the alignment and the introduction of the iron may occur concurrently. In  706 , the elements are pressed against the iron to cause surface melting of both elements. At  708 , the elements are separated. At  710 , the iron is removed from between the elements. At block  712 , the slightly molten surfaces are pressed together and at stage  714  a new partial object including both sub-elements then exists. In other embodiments, the probe between the elements may introduce a glue or adhesive or additional molten plastic resin to cause the adhesion between the elements instead of heating the element to melt them. 
       FIG. 8  is an internal view of one embodiment of the invention with a mill arm extended. A mill arm  802  is shown extended to allow it to detail a partially assembled object within the assembly zone  200 . Mill arm  802  can typically be driven in arc to provide access to the entire object under assembly. Mill arm  802  also includes a drive to adjust the incidence angle of the mill head  804 . Typically, provided that mill head  804  can reach the center of the turntable that provides build surface  208 , mill bit  804  can be used to detail any portion of the object simply by appropriately rotating the build surface  208 . In one embodiment, mill arm is provided with a suction head to collect waste particles that result from the milling. In another embodiment, an air jet is provided to clear the particles from the assembly area into a collection tray below. This avoids the risk of particles from the milling causing defects in subsequent welds. 
     While layers are being added within the assembly zone, typically mill arm  802  will be retracted. However, milling may be performed while extrusion is occurring within the fabrication zone. Thus, layer-by-layer access to the object can be provided to mill head  804  such that portions of the object that might be obscured when completed can be correctly detailed during the assembly of the object. The mill is driven by the motor  806  and controlled by the internal processor (not shown). In this way, minor defects in the extrusion fabrication may be corrected by subtractive detailing with mill head  804 . Additionally, edge detail may be provided to permit a thicker layer to be molded than would be possible if the extrusion needed to provide all the edge detail directly. Thus, the extrusion of a layer can be used to “get close” and the mill head  804  can be use to provide added precision. 
       FIG. 9  is a schematic diagram of one embodiment of the invention. In one embodiment, plural fabrication zones  902  and  904  are associated with one assembly zone  900 . Layers or sub-elements may be manufactured and assembled in an interleaved manner to improve the speed of overall fabrication. In some embodiments, a camera  910  may exist within the assembly zone. The camera may be linked to a wide area network (WAN)  914  such as the Internet by link  912 . Link  912  could be a wired or wireless link and may be direct or through a host processor. A remote node  916  may the access the camera feed in substantially real time to monitor the assembly process. Remote node  916  could be a desktop computer, a smart phone, a tablet computer, a laptop, or any suitable WAN connected device. Some embodiments may include cameras in the fabrication zone(s) to permit similar remote monitoring of the fabrication process in addition to or instead of the assembly process. 
     Remote node  916  may also provide an interface that permits a user to send control signals back to the fabricator to control its operation, including for example starting or stopping the process, adjustment of system calibration, etc. In one embodiment, the interface is a web page served to the remote node  916 . In some embodiments, no control panel exists on the fabricator itself and all control of the operation is performed though the interface on remote node  916 . It is also within the scope and contemplation of the invention to have plural assembly zones in addition to plural fabrication zones. In some embodiments, one or more layers may be assembled in a first assembly zone and then added to other layers that have been previously assembled in the second assembly zone. By increasing the parallelism of layer production and assembly, output speeds can be increased. 
       FIG. 10  is a schematic diagram of an embodiment of the invention permitting modular expansion. An array of additive fabrication cells  1002 , which may be each similar to the fabrication zone described with reference to  FIGS. 1-5  above, may be modularly added. In other words, a user may initially constitute the system with fewer than the 4×4 array of additive cells  1002  and then expand the system as demand requires to, in this example, achieve a maximum of 16 additive cells  1002 . Similarly, each additive cell  1002  may have a corresponding subtractive cell  1004 , such that a layer fabricated in an additive cell  1002  is transported to the adjacent subtractive cell for detailing, for example, with a mill head, before being transported by lateral transport  1006  to elevator  1008 . 
     Elevator  1008  raises the individual platen  1010  and places it in its intended order within the platen hopper stack  1012  residing in hopper  1014 . The bottom platen in the platen stack  1012  is transported into assembly zone  1018  and aligned with build object  1016 . Proper alignment of the platens can be assured by registration holes and pins that guarantee a known orientation. The weld layer  1020  is then welded within the assembly zone  1018  in a manner similar as described above in connection with  FIGS. 6A-C . Specifically, a suitable hot plate heats the surface of both build object  1016  and weld layer  1020 . The hot plate is then removed and the layers are pressed together. In this manner, because no post-weld detailing is required, the object can be assembled layer by layer as quickly as the welding can occur. 
     Once the layer is removed from its platen, the platen itself can be transported back laterally to the elevator shaft which will return it to a lateral transport  1006 , which will return it to a waiting additive cell for extrusion of a subsequent layer. While in the shown embodiment a 4×4 array of additive and subtractive cells is shown, it is envisioned that other embodiments of the invention may permit either larger or smaller arrays of cells. Moreover, it is also envisioned that an additional elevator hopper and assembly zone may be added, for example, to the opposite end of the fabrication array such that two objects may be built concurrently. Notably, because the creation of layers in the additive cells, the detailing of layers in the subtractive cells and the addition of layers in the assembly zone can all occur in parallel, higher speed object creation is rendered possible. 
     In some embodiments, assembly zone  1018  is sufficient to accommodate the build of an object that has layers larger than any of the additive cells  1002  can draw at one time. This will generally imply that edges of layer sub-element should be welded together. However, by appropriately selecting the sub-elements of subsequent layers such that vertically pressed lamination occurs, the need for a side pressure process can be eliminated. For example, presume a cylindrical object for which an additive cell  1002  can only produce a third of the cylinder. If the three pieces forming each layer are shifted ten degrees on each subsequent layer, the weak joint between the sub-elements of any single layer does not cause systemic weakness in the finished object. 
       FIG. 11  is a flow diagram of operation of one embodiment of the invention. At block  1102 , material from which a three-dimensional object is to be built is loaded into a fabricator. At block  1104 , geometric data corresponding to the object to be built is received in the fabricator. Geometric data may have been processed prior to loading to expressly indicate the sub-elements in order of build, or they may be processed internally within the apparatus from a simple three-dimensional model. At block  1106 , a layer of the object to be built is molded onto a receiving plate in a fabrication zone of the apparatus. That layer is then transported and bonded to the object being formed at block  1108 . In the case of the first layer, it is bonded to a build surface and forms the substrate for subsequent additional layers to be bonded. A determination is made at decision block  1110  whether subtractive detailing is required. If subtractive detailing is required, the object is milled to subtractively detail the layer at block  1112 . The determination is made at block  1114  whether the object is complete. If the object is complete, the object is ejected from the fabricator at block  1116 . Otherwise, a next layer is molded and bonded and the process continues. It should be understood that, while this is shown as a linear flowchart, many of these operation can occur in parallel. In particular, it may be possible to mill one layer while extruding another or, depending on the number of extrusion zones and the parallelism permitted, extruding and bonding may occur concurrently. These levels of parallelism are intended to be within the scope and contemplation of the invention. 
     While embodiments of the invention are discussed above in the context of flow diagrams reflecting a particular linear order, this is for convenience only. In some cases, various operations may be performed in a different order than shown or various operations may occur in parallel. It should also be recognized that some operations described with respect to one embodiment may be advantageously incorporated into another embodiment. Such incorporation is expressly contemplated. 
     It should be appreciated that reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Therefore, it is emphasized and should be appreciated that two or more references to “an embodiment” or “one embodiment” or “an alternative embodiment” in various portions of this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined as suitable in one or more embodiments of the invention. 
     In the foregoing specification, the invention has been described with reference to the specific embodiments thereof. It will, however, be evident that various modifications and changes can be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.