Abstract:
A energy absorbing structure is disclosed for the purpose of protecting human occupants of a vehicle from the damaging effects of sudden accelerative or decelerative forces by means of plastic deformation of the structure. The structure consists of a generally rectangular sheet of a stranded material, preferably expanded metal, which has nonisotropic properties and which is employed in the construction of the bottom or back of a seating structure for the vehicle occupant. The nonisotropic properties of the sheet allow it to be designed in such a way that it resists deformation when exposed to any force below a specified threshold; beyond the specified threshold, the sheet absorbs the force through plastic deformation, thus attenuating damaging force to which the occupant may be exposed. Such a device may be used in the seating apparatus of various vehicles, including aircraft, automobiles, space vehicles, helicopters, heavy machinery, tractors, military vehicles, and the like.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This patent application claims priority based upon the following provisional patent application: Ser. No. 60/116,822, filed on Jan. 21, 1999, of the same or similar title. 
    
    
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     This invention was made under the auspices of Bergey Aerospace, Inc. No rights will accrue to any other entity, either governmental, educational, or commercial. 
     BACKGROUND 
     A. Field of the Invention 
     The present invention relates generally to shock-absorbing structures, more particularly, to safety features incorporated into vehicle seats, namely those found in aircraft, to protect the occupant, insofar as it is possible, from shock induced injury. Such structures find utility in related fields such as automotive, machinery, or any vehicle having human occupants. 
     B. Description of the Related Art 
     Designers of vehicles have strived for many years to provide safety devices designed to protect the vehicle occupants from injury in the event of a mishap. Such vehicles include land based vehicles such as automobiles, trucks, earth moving equipment, and the like; sea based vehicles such as boats and submarines; and flying vehicles such as aircraft, helicopters, and spacecraft. All such vehicles feature a to compartment in which the occupants, consisting of the operator and passengers, are seated and surrounded by instrumentation and apparatus for controlling the vehicle. Because the vehicle is generally in motion, external forces may cause the compartment to abruptly change its direction of motion, resulting in forces exerted upon the occupant. If such forces are too great, the occupant can be seriously injured or killed from these forces. 
     Various means have been proposed to protect the occupant from such forces. Seat belts are a notable example of such protection. They prevent the occupant from being thrown out of the seat and against the instrumentation, control apparatus, or the boundaries of the occupant&#39;s compartment, thus injuring the occupant. However, the seat belt is generally inelastic; that is, when the occupant is abruptly thrown against the seat belt, the force is absorbed by the human body against the seat belt. If the force is too great, then the human body is unable to dissipate the force by its natural elasticity and the tissue or bones will break down, resulting in injury. Some inventors have made an effort to dissipate the force by adding an elastic component to the seat belt which permits the belt to expand somewhat. For example, U.S. Pat. No. 4,060,276, by Maeyerspeer, achieves such elasticity by adding slots in the belt which allow the belt to tear when presented with a specified force to thus absorb the energy of the force. 
     Other devices protect the occupant by deforming when the occupant is forced against the device. Thus, while seat belts prevent the occupant from being thrown out of the seat, they do nothing to protect the occupant from vertical components of the impact forces such as those commonly experienced in airplane or helicopter accidents. Given that the occupant is in a normal seated position, such forces might be caused by a vertical fall ending in a sudden stop. Cushions, springs, and hydraulic shock absorbers, all installed in the base of the seat, have historically been used to dissipate such sudden and short duration forces. Some of devices exhibit the tendency to return to the original configuration after dissipating the force, resulting in an equal and opposite reaction and allowing the device to resume its shock-absorbing function at a later time. 
     Le Mire (U.S. Pat. No. 3,586,131) teaches the use of a mechanical energy absorbing device permitting absorbing a thrust by plastic deformation. The device consists of a metal plate having a central fastening point and a plurality of lateral-fastening points. The metal plate has staggered arcuate apertures concentrically arranged around a single centrally-located fastening point so that the plate can expand in tension when subjected to a load tending to move the central fastening point away from at least one of the lateral-fastening points. This device is designed to deform equally in all directions and is thus believed to exhibit isotropic characteristics under load. Le Mire teaches its use for securing or coupling automotive parts likely to be moved or struck by the passengers of a vehicle in case of a violent mishap. 
     Simpson et. al. (U.S. Pat. No. 5,338,090, Aug. 16, 1994) features a leg structure in a seat frame which deforms to absorb energy. Marechal (U.S. Pat. No. 5,499,783, Mar. 19, 1996) describes device in which energy is dissipated by having a male part which penetrates by force and with plastic deformation into an aperture of a female part, where both parts are components of an underframe for a passenger aircraft seat. 
     Audi et. al. (U.S. Pat. No. 5,700,545, Dec. 23, 1997) describes an energy absorption device using an expanded metal structure sandwiched between an incident surface and a basal surface, wherein the expanded metal structure is oriented substantially perpendicular to the incident surface. This device is designed so that a force striking an incident surface will cause the expanded metal structure to collapse through plastic deformation during energy absorption so that damage to the object protected by the basal surface is prevented. It is interesting to note that the ends of the strands in this structure are unconstrained, which means that an impacting object deforms the strands in compression rather than in tension, which further facilitates collapse of the structure. The inventors teach the use of this structure for head rests, arresting gear, landing gear, but not for seating arrangements. 
     Aircraft present unique problems in the design of energy-absorbing structures such as seats. First, weight must be kept to a minimum so that the aircraft may be efficiently operated. Cushions, springs, energy attenuation devices, and hydraulic shock absorbers all have a certain non-trivial and adverse weight and space effects that detract from the utility of the aircraft and other vehicles, thus requiring adverse design tradeoffs that may reduce efficiency of the aircraft. Second, Title 14 of the Code of Federal Regulations, which governs aircraft certification, requires dynamic seat tests, to check the ability of the seat to prevent injury to the occupant when the seat is exposed to associated forces with specified deceleration rates. Cushions, springs, hydraulic shock absorbers, or complex seating frames which deform on impact, most of which operate through compression upon loading, must be substantially constructed in order to provide the necessary protection to the occupant; the resulting construction enhancements and considerations invariably result in increased weight. 
     Thus, it would be desirable to find an efficient and economical means of seat construction for aircraft and helicopters that will accomplish the following: 
     1. Support the occupant of an aircraft seat comfortably and without permanent deformation during all normal operations within the approved structural envelope of the aircraft. 
     2. Absorb energy and thereby attenuate the decelerative forces imposed on the occupant at the deceleration rates and loading angles specified by law and regulation (i.e. the Code of Federal Regulations, Title 14) or in other formal or informal requirements for occupant safety and protection. 
     3. Exhibit flexibility in configuration so that the point at which plastic deformation occurs can be controlled. 
     4. Have low weight for aircraft use. 
     5. Be inexpensive to construct and maintain. 
     Such considerations as low weight, flexibility in configuration, and energy absorption, and expense would also have utility in seat construction for vehicles other than aircraft and helicopters. 
     SUMMARY 
     It is therefore an object of the present invention to provide ab energy-absorbing structure which will absorb shock forces experienced by a human being in a seated position within an airborne vehicle. 
     Another object of the invention is to provide an energy absorbing structure which is light weight. 
     Another object of the invention is to provide a means of absorbing said shock without the accompanying rebound exhibited by certain conventional energy-absorbing designs. 
     Another object of the invention is to provide an energy absorbing structure which absorbs forces impacting the structure primarily by means of loading the structure in tension rather than loading the structure in compression. 
     Another object of the invention is to provide a simple and lightweight energy-absorbing structure which will meet the certification requirements of the United States Federal Aviation Agency as documented in Part 23 of Title 14 of the Code of Federal Regulations or other requirements that may be promulgated by the United States Government from time to time. 
     Another object of the invention is to provide an energy absorbing structure which is easily constructed from materials well known to the aviation industry. 
     Another object of the invention is to provide an energy absorbing structure which is relatively inexpensive to build and maintain, as compared with the existing art. 
     Another object of the invention is to provide an energy absorbing structure which can be supported by a rigid supporting means. 
     These objects of the invention are achieved by a special seat pan supported by a seating means which provides a space in which the seat pan can expand when subjected to a load normal (i.e., perpendicular) to the incident surface of the seat pan. In the case of an aircraft seat, the seating means provides the structural interface between the seat pan and the cabin structure of the aircraft. The seat pan is formed from a stranded structure which is formed from etched, cast, joined, mechanically perforated, or other forming processes, such structure being formed so as to exhibit nonisotropic characteristics. The best embodiment in this application is an expanded metal structure where the metal has been slit and partially expanded in a manner which will permit the sheet to expand when deformed by a rigid body, namely the body of the occupant when that body is subjected to forces against the seat pan. The slits in the seat pan are designed to deform plastically and permanently under shock loads, so that the seat pan as a unit expands with the force when such loads exceed a given, predetermined limit. This limiting point beyond which plastic deformation occurs is determined by a number of design factors, such as the two-dimensional shape of the seat pan, the manner in which it is supported at its perimeter, the orientation of the apertures in the expanded metal with relationship to the supporting points, the material from which the seat pan is made, and the dimension and shape of the apertures in the seat pan. This plastic deformation will absorb energy and reduce the decelerative forces acting on the occupant of the seat without the reactive component which would tend to return the seat pan to its unexpanded configuration. Such a seat pan is manufactured from light weight materials. Since it replaces a normal component of the seating structure, it does not occupy excessive space or add materially to the weight of the seat structure. The seat pan may optionally be covered with a soft, resilient pad made of such materials as polyurethane, rubber, or cloth materials, in order to increase comfort for the occupant and to more evenly spread the load subjected to the seat pan. 
     Prior seat designs achieve their energy-absorbing characteristics through plastic deformation of the seat supporting means, whereas the invention described in this disclosure achieves its energy-absorbing characteristics through plastic deformation of the seat pan itself supported by a rigid supporting means. It is believed that this aspect of the invention is unique and novel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a perspective view of a seating structure incorporating the invention and which is representative of the invention in general. 
     FIG. 2 shows a side view of the invention as it would normally be configured before energy absorption through plastic deformation. 
     FIG. 3 depicts a side view of the invention after it has been subjected to a load which would cause the invention to plastically deform and expand into the space below. 
     FIG. 4 shows a detailed view of one embodiment of how the invention is connected to seating means. 
     FIG. 5 is a side view of the seating structure of FIG.  1 . 
     FIG. 6 illustrates a representative aperture formed of two strands in a typical stranded structure made of slitted metal. 
     FIG. 7 shows five different embodiments of the methods of constraining the strands in a shaped seat pan and providing support thereof. 
     FIG. 8 shows a bottom view of one embodiment of how a seat pan is supported so as to constrain the strands within the structure. 
     FIG. 9 shows a representative force-deflection graph illustrating the principle whereby the invention operates. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention is best understood by describing it in a typical application which is shown for illustration purposes only and is not intended to limit application of the invention. Referring to FIGS. 1 and 5, we see one such application in which the invention is employed as an aircraft seat. An aircraft seat  10  is shown, consisting of a back portion  15 , a bottom portion  30 , and a support means  40 . Back portion  15  is rigidly connected to the bottom portion  30 , although the capability of reclining back portion  15  may be built into the structure for added comfort to the occupant by pivotably connecting back portion  15  to bottom portion  30  by hinge means  35  at their juncture. Back portion  15  provides support for the occupant&#39;s back, shoulders, and head, the back portion  15  having a generally rectangular back support frame  20  constructed of tubing, webbing, springs, straps, and the like, and covered with a soft, elastic back cushion material  25  for comfort to the seat occupant. Bottom portion  30  supports the thighs and buttocks of a human torso in a seated position (not shown). Both back portion  15  and bottom portion  30  are positioned with respect to each other and supported by support means  40  which connects the aircraft seat  10  to the cabin floor  55  of the aircraft. Bottom portion  30  consists of a seat pan  60 , shown with a slight curvature for comfort, and a seat cushion  75  upon which a human torso is seated. Support means  40  may be of any suitable construction which supports the back portion  15  and positions bottom portion  30  a distance from floor  55  to permit seat pan  60  to unobstructedly expand into space  80  when vertical force  85  is exerted upon a human torso seated upon the bottom portion  30 . When subjected to vertical force  85 , a seated human torso (not shown) is downwardly pressed against cushion  75  which transfers force  85  to seat pan  60 , thereby causing seat pan  60  to expand downwardly through plastic deformation of seat pan  60  to arrive at pan position  65  as the kinetic energy of force  85  is dissipated. Seat pan  60  provides a generally horizontal base for cushion  75  which cushion is constructed of a suitable elastic material which provides a soft and comfortable interface between seat pan  60  and the human torso (not shown). Seat pan  60  is held in a horizontal orientation at its two opposing sides by front frame member  45  and rear frame member  50 . 
     Seat pan  60  is secured to frame members  45  and  50  by partially wrapping the material of seat pan  60  around said frame members and securing the material from the bottom side, as best shown in FIGS. 2 and 3. Bight  70  is formed in each of two opposing sides of seat pan  60  for capturing a frame members  45  and  50  therein, so that the material of seat pan  60  is in contact with a generally semicircular portion of frame members  45  and  50 . As best seen in detail in FIG. 5, bight  70  is secured to a frame member from the underside by means of a rectangular bar  90  which captures the leading edge  72  of seat pan  60  between it and frame member  45 ,  50  and frictionally holds the seat pan therebetween. A plurality of screws  55  inserted through rectangular bar  90  and into threaded holes in respective frame member  45 ,  50  are tightened so as to maintain rectangular bar  90  in frictional contact with both leading edge  72  and the respective frame member  45  or  50 . The degree of frictional contact may be adjusted in this manner so that a sufficient force  85  (FIG. 5) exerted upon seat pan  60  will cause the side to slightly travel back around frame members  45 ,  50 , thus augmenting the energy-absorbing capability of seat pan  60 . This manner of securing seat pan  60  to frame members  45 ,  50  has the advantage that all points along the leading edge  72  of seat pan  60  can be simultaneously be constrained by bar  70  with an adjustable, uniform pressure. However, this is but one embodiment, and seat pan  60  may also be rigidly secured by welding, rivets, flanged screws, bolts, springs, or any similar means without departing from the scope of the invention. 
     The support means  40  is shown in FIG. 1 as a generally rectangular frame with tubular members, primarily for illustration of the energy absorbing concept of the invention. However, any method may be used to support seat pan  60  a distance from the nearest surface so that it may freely expand into the space therebetween. In small aircraft, portions of the aircraft structure may perform double duty both as structural components and as an anchoring point for one or more sides of seat pan  60 . For example, in small aircraft, the main wing spar runs through the cabin and may be utilized as one of the supporting members of the rectangular frame. Similarly, the side wall of the aircraft cabin may provide a means for anchoring one side of seat pan  60 . Also, members having cross sections other than circular may be used to suitable construct support means  40 . If a generally rectangular frame composed of elongate members is used, then embodiments of frame members other than those illustrated as  45 ,  50  may be used; for example, frame members having triangular, rectangular, or elliptical cross-sectional shapes may also be used without departing from the scope of the invention, provided that they are suitable for the particular purpose of providing support for the seat  10 . 
     Support means  40  has been heretofore described as being a rigid structure, but this should not preclude the use of other energy absorbing seating frames described in the prior art and in conjunction with the present invention. A distinction should be made here between a “rigid” support means and “rigid” support of the seat pan. The former phrase refers to the fact that energy-absorption is primarily being accomplished by the seat pan and not the support means, whereas the latter phrase refers to fact that the seat pan is kept in the same relative relationship with the support means and the surface beneath the seat pan. As will be presently explained, the latter term also refers to the constraint of strands in the expanded metal composition of the seat pan at the supporting points so they are placed in tension by force  85 . 
     The design and construction of seat pan  60  involves a multiplicity of factors in order to provide an apparatus with desired shock-absorbing characteristics, which are described in a load-deflection graph having a typical aspect as given in FIG.  9 . Here the magnitude of force  85  (FIG. 5) is given in pounds on the vertical axis  210  and the deflection in inches of seat pan  60  as it is deflected by force  85  to position  65  is given by the horizontal axis  215 . Loads less than value  235  will result in an elastic deflection of the seat pan; the seat pan will elastically return to its rest state when the load is removed. The range of deflection for such loads is called the elastic range. However, for loads exceeding value  235 , the seat pan will deflect plastically and permanently and will not return to its rest state when the load is removed. The design task for a suitable seat pan is to configure the seat pan for a selected threshold value  235  and a selected deflection value  240  which may be accommodated by the space  80 . 
     The structure considered for use in the seat pan  60  of the present invention and considered to be the best embodiment is a sheet of slitted metal which has been formed by piercing the metal with parallel rows of alternating slits and then partially expanding the sheet so that the slits form hexagonal-shaped openings, or apertures  100 , in the sheet, as indicated in FIG.  6 . The same slitted metal material  21  used for seat pan  60  could optionally be used in back frame  20 . The slitted metal material for seat pan  60  exhibits nonisotropic properties. The term nonisotropic is well-known in the art of materials science and, as used herein, describes a characteristic of a material whereby the properties it exhibits in a first direction of orientation differ from the properties it exhibits in a second direction. The second direction is most commonly oriented perpendicular, or normal, to its first direction. The nonisotropic property of slitted metal results from the choice of metal alloy; the expanded pattern in which the material is physically manufactured, constructed, or configured; the degree of expansion of the material; the orientation of each aperture of the material with respect to how the seat pan is supported; and the number of layers of such material. 
     A typical aperture  100  in a sheet of expanded metal is shown in FIG.  6 . Aperture  100  is formed of two parallel strands  115   c ,  115   d  and  115   e ,  115   f  which are periodically joined to adjacent strands at a bond  110   a  or  110   b , and with each other at bonds  110   c  or  110   d . Aperture  100  is seen to have two axes, a long way of opening  125  which is colinear with the slits and a short way of opening  120  which is perpendicular to the slits. As shown, strands  115   c ,  115   d  and  115   e ,  115   f  run horizontally. When the ends of said strands are fixed in place at the perimeter of seat pan  60  and a force  85  applies load to the seat pan, then aperture  100  tends to close, i.e. the short way of opening  120  becomes smaller and the long way of opening  125  becomes longer. Note, however, that cross strands may be conceptualized running vertically through the structure. As shown in FIG. 6, such a cross strand might be formed of segments  115   b ,  115   d ,  115   f ,  115   h  or of segments  115   a ,  115   c ,  115   e ,  115   g . If the ends of a cross strand are fixed in place at the perimeter of seat pan  60  and a force  85  applies load to the seat pan, then aperture  100  tends to become more opened, i.e. the short way of opening  120  becomes longer and the long way of opening  125  becomes smaller. In either event, the strands and cross strands, each having their ends constrained, are placed in tension by an applied load. The interplay between strands and cross strands is not completely understood and can become quite complex depending upon the way in which these strands and cross strands are constrained at the perimeter of the seat pan by supporting means  40 . 
     As illustrated in FIGS. 1,  2 ,  3 ,  5 , and  8 , seat pan  60  is shown having support by supporting means  40  at two opposed sides along its perimeter. However, other methods of supporting seat pan  60  (and thus constraining selected strands and cross strands) may be used depending upon the desired expansion characteristics required to provide a given quality of energy absorption. FIGS. 7 a  through  7   e  show a number of different support methods, depending upon the shape of seat pan  60 . 
     FIGS. 7 a  and  7   b  show two different methods by which seat pan  60  having a rectangular shape is supported by a single support along two opposed sides. Note in FIG. 7 a  that the expanded material comprising seat pan  60  may be oriented either with the long way of opening in the direction from support  130  to support  135  or the short way of opening in the direction from support  130  to support  135 ; the load-deflection curve of each orientation is radically different. 
     FIGS. 7 c  and  7   d  show two different support methods by which a seat pan has four points of support, one on each of its four sides. The seat pan in FIG. 7 c  is supported at  150 ,  155 ,  160 , and  165 , and the seat pan in FIG. 7 d  is supported at  170 ,  175 ,  180 , and  185 . However, the lengths of the sides in FIG. 7 d  differ from those in FIG. 7 c . Again, the apertures of the seat pan in FIG. 7 c  can be oriented so that either the long way of opening or the short way of opening is in a direction from support  150  to support  165 . The same general rule holds true for FIG. 7 d.    
     FIG. 7 e  depicts a support scheme whereby two supports are provided along each of two opposed sides, again providing four points of support identified by  190 ,  195 ,  200 , and  205 . 
     The material chosen for the expanded metal sheet of a seat pan and considered to the best embodiment for use in seating applications is an aluminum or steel alloy with sufficient elongation prior to failure to allow a high degree of plastic deformation. In this application both aluminum and steel alloys have the desirable properties of light weight, ease of forming, sufficient strength to support desired loads, low cost, and fire resistance. These properties make the material useful in seating applications involving aircraft, helicopters, and automobiles. 
     Any seating apparatus for use in aviation applications must meet certain standards under the current regulations (Title 14 of the Code of Federal Regulations, or 14 CFR.) governing the United States Federal Aviation Administration (FAA). In particular, Part 23 of the FAA regulations requires that the seat design must be capable of surviving an impact of not less than 32 feet per second at 60 degrees positive pitch while peak deceleration must be at least 19 g and within 0.05 seconds after impact. A seating design employing a seat pan manufactured according to the principles herein described can be configured, as shall be presently be seen, such that it plastically deforms so as to absorb the indicated load but at lower values it elastically keeps its shape and configuration. The range of deflection for a nonisotropic material is determined by the orientation of the applied load. The configuration of the seat pan is chosen so that it returns elastically to its normal shape for loads within the normal operating range of the vehicle and plastically deforms for values in the plastic range for loads greater than the normal operating range, the seat pan deforms plastically, thereby absorbing energy and reducing the load applied to the occupant. It was found during testing that the energy-absorbing characteristics of the seat pan could be empirically tailored to specific requirements by varying (1) the composition of the material, (2) the thickness of the material, (3) the shape of the apertures, (4) the orientation of the apertures, (5) the method of attachment of the seat pan to the support means, and (6) the shape of the seat pan. 
     In a test case, an energy absorbing seat pan was designed and fabricated with the proper selection of material and a shape, as described in this disclosure, to meet the impact attenuation requirements given above. By extension, the design parameters can be selected to meet a wide variety of occupant protection requirements. 
     Seat pans with various combinations of pan material, pan shape, aperture size, and aperture orientation were tested statically to establish the load/deflection and energy absorbing characteristics of such combinations when subjected to loads beyond the elastic limit. Support members  45  and  50  of the seat pan in FIG. 1 were located at a distance of 16.0 inches from centerline to centerline. The seat pan  60  was attached to support members  45 ,  50  as shown in FIGS. 2 and 4. In order to represent expected conditions of use, the seat pans were covered with various combinations of polyurethane and polyethelene foam. Tests were conducted using an anthropometric loading shape to represent the loads exerted by a human torso. 
     The static tests indicated that there were two preferred combinations of parameters that provided the desired load/deflection characteristics. In both cases, the seat pan was fabricated from 1004/1006 expanded low carbon steel. The thickness of the material was 0.034 inch and the opening size of the expanded metal was 0.40 inch (short way of opening) by 0.80 inch (long way of opening). The width of the pan was 15.0 inches. In the first test case, the orientation of the apertures mesh was with the long way of opening in the direction of the supporting structure. In the second test case, the orientation of the apertures was with the short way of opening in the direction of the supporting structure. 
     Dynamic tests of the seat pan and support structure were performed on an impact sled at the Federal Aviation Administration Aeromedical Center in Oklahoma City, Okla., under conditions meeting the testing requirements of Part 23 of 14 CFR. The anthropometric dummy used for the tests represented a 50 percentile male and was fitted with load measuring devices to record spinal loads. Both aperture orientations were tested. The seat pan configured according to the second test case, with the short way of opening running in the direction of the supporting points, was found to be successful in meeting the 14 CFR requirements when the width of the seat pan was tapered from 15.0 inches at the rear support member  50  to 10.0 inches at the forward support member  45 . This preferred embodiment as described above is shown in FIG. 7 b . Tests of the orientation of the first test case confirmed that the dynamic response characteristics of the seat pan could be varied by the selection of aperture orientations, as well as the other design parameters noted above. 
     Actual deformation mechanics on a microstructural or finite element analysis of this energy-absorbing structure are extremely detailed and complex. It should be recognized that the disclosed invention encompasses all possible configurations of slitted metal and deformation mechanics. While only a preferred embodiment has been illustrated and described, obvious modifications may be made within the scope of this invention and the claims without substantially changing its functions. Accordingly, the scope of the invention should be determined not by the embodiments illustrated but by the appended claims and their legal equivalents.