Abstract:
Disclosed is an autonomous radar guidance of an otherwise radar-directed projectile (RDP). The preferred embodiment uses an inexpensive radar receiver with an inexpensive slow wave antenna, placed internally in a gun projectile, and on the surface of the projectile, respectively. The receiver detects the angle and range of the target relative to the body coordinates of the projectile. The radar receiver operates as a bistatic radar apparatus with the primary illumination emanating from the fire control radar directing the fire of the gun. When integrated with an on-board trajectory correcting system, such as divert thrusters of miniature proportions, the projectile autonomously refines its otherwise ballistic trajectory to the target. The trajectory refinements produce improved kills per round, with the potential for reducing the ammunition expended and time-loading on the fire control system and its guns. An alternative embodiment, in addition to receiving the target-reflected fire control emissions, receives and processes the fire control radar emissions directly in order to enhance homing accuracy.

Description:
CROSS-REFERNCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of provisional patent application No. 60/284,004 filed Apr. 16, 2001, of Knut Sverre Kongelbeck and Ada Mendelovicz, entitled “RADAR DIRECTED PROJECTILE,” the contents of which are hereby incorporated by reference in its entirety for all purposes. 
     
    
     FEDERALLY SPONSORED RESEARCH  
       [0002]     Not applicable.  
       BACKGROUND—TECHNICAL FIELD OF THE INVENTION  
       [0003]     This invention relates to guided projectiles in general and more particularly to the method and apparatus for the autonomous radar guidance of an otherwise radar-directed projectile (RDP).  
       BACKGROUND—PRIOR ART  
       [0004]     The prior art pertaining to guided projectiles includes the fire control/weapon systems in which they may be implemented, the projectiles themselves, their guidance systems and the constituent on-board sensing elements and supporting data processing and trajectory correcting means that enable autonomous projectile homing.  
         [0005]     U.S. Pat. No. 5,669,581 issued Sep. 23, 1997 to H. N. Ringer and assigned to Aero-Jet General Corporation discloses a spin-stabilized projectile for destroying a distant target using the projectile&#39;s spin to carry out functions beyond stabilization such as target imaging, course-correction, and warhead-aiming. The Ringer patent discloses the use of diverting thrusters and describes the use of an imaging array of infrared detectors for scanning images and pitch thrusters for pitch control.  
         [0006]     U.S. Pat. No. 5,425,514 issued Dec. 29, 1993 to V. A. Grosso and assigned to Raytheon Company discloses a spin-stabilized projectile using a directional thruster nozzle acoustically coupled to the guidance electronics in the nose section for control of coning, nutation, precession and lateral maneuverability via angle-of-attack. The Grosso patent further discloses a laser illuminator driven by a fire control system whereby the projectile&#39;s optical (infrared) telescope develops a boresight error to be used in steering and also discloses an alternative embodiment of a purely aerodynamic type with a screw-on wrap-around tail-section and a screw-on seeker and guidance section.  
         [0007]     U.S. Pat. No. 5,685,504 issued Nov. 11, 1997 to A. J. Schneider, et al. and assigned to Hughes Missile Systems Company discloses a system using off-board target laser illumination and tracking together with off-board projectile laser tracking to generate and then up-link to the projectile the necessary divert commands. The Schneider patent further discloses the use of either aerodynamically or thruster controllable and maneuverable projectiles.  
         [0008]     U.S. Pat. No. 5,788,178 issued Aug. 4, 1998 to R. F. Barrett discloses a small caliber laser-guided bullet of negligible axial spin having a self contained guidance system including on-board laser sensors and navigational circuits capable of detecting a laser target signature, determining the deviation of the bullet from an optimal trajectory along which the bullet would impact a hostile target, and generating an electrical signal to piezoelectric steering control surfaces to effect a change in the course of the bullet. Barrett further discloses the use of logic circuits to process sensor pulses into steering commands of a 0.5 caliber (12.7 mm) projectile fired from a smooth bore barrel.  
         [0009]     U.S. Pat. No. 5,529,262 issued Jun. 25, 1996 to T. G. Horwath discloses a guidance seeker for a spinning projectile defined by its angular momentum vector, comprising a lens system or a millimeter-wave antenna for receiving radiation derived from a target and forming an image of the target. Horwath further discloses a thruster control scheme and a multimode sensor configuration.  
         [0010]     U.S. Pat. No. 5,392,393 issued Jul. 9, 1968 to H. W. Ehrenspeck discloses an endfire antenna having surface wave propagation along the longitudinal axis thereof and being tunable to maximal gain for any preselected frequency within a predetermined frequency range. The endfire antenna disclosed includes a transverse feed and endfire reflector with an associated guide element. Ehrenspeck further discloses a dielectric rod endfire antenna of rectangular shape in cross-section and linearly tapered in thickness in the longitudinal axis.  
         [0011]     U.S. Pat. No. 4,591,865 issued May 27, 1986 to Y. S. Canal et al. and assigned to Philips Corporation discloses a rectangular parallelpiped bidirectional antenna, containing a dielectric material and having a metal-plated surface that is designed to radiate decimetric or centrimetric waves. The antenna is disclosed as exhibiting highly directional patterns with low sidelobes and to be of simple construction.  
         [0012]     U.S. Pat. No. 6,137,453 issued Oct. 24, 2000, to J. J. H. Wang, et al. and assigned to Wang Electro-Optical Corporation discloses a broadband, miniaturized, slow-wave antenna for transmitting and receiving radio frequency signals. The slow-wave antenna comprises a dielectric substrate with a traveling wave spiral structure mounted on a surface, and a conductive surface member mounted on the opposite surface.  
         [0013]     U.S. Pat. No. 4,925,129 issued May 15, 1990 to D. Salkeld, et al., and assigned to British Aerospace, PLC, discloses a missile defense system for guiding a subprojectile to intercept and destroy a target that includes a radar tracker to acquire and track the target, predictor means for determining the predicted track of the target, and a guided projectile for being guided onto a rear reciprocal track of the target. The patent also discloses a guided projectile that includes an optical tracker for acquiring and tracking the target at relatively small distances and a sub-projectile for being guided on the line-of-sight from the optical tracer and the target.  
         [0014]     U.S. Pat. No. 4,476,785 issued Oct. 16, 1984 to Hoffman, et al., and assigned to Mauser-Werke Oberndorf GmbH discloses a hood covering for a subcaliber projectile. U.S. Pat. No. 5,841,059 issued Nov. 24, 1998 to P. Laurend, et al., and assigned to Luchaire Defense, S.A., discloses a laser-homing subcaliber projectile that is a spinning, parachuting projectile that detects and fuzes on target-reflected laser emissions.  
         [0015]     Modernly, a fire control radar with a radar-directed gun can be explained by example with the Phalanx Close-In Weapon System (CIWS). The Phalanx CIWS is a point defense weapon that fires 20 mm projectiles at a rate of around 3,000 to 4,500 rounds per minute in bursts of 60 to 100 rounds. The gun system autonomously acquires and engages the target. The radar tracks the stream of bullets and the target. The aim of the gun is corrected continually so that the stream of bullets will intersect the target.  
         [0016]     The lethality of a radar-directed gun and the Phalanx in particular could be improved by adding a capability to its projectiles for autonomous guidance. Meaning that should a projectile determine or otherwise estimate that it was about to miss its target, it could divert its trajectory to correct for the miss. The system benefit of this capability would be to reduce the number of projectiles necessary to destroy a target and an associated unburdening of the fire control system. In addition, by increasing the probability of a given round successfully destroying its intended target, the effective lethal range of a radar-directed gun like the Phalanx could be increased. The benefit of the extended lethal range is the improved survivability of the Phalanx-hosting platform.  
       SUMMARY OF THE INVENTION  
       [0017]     Disclosed is a projectile for destroying, disabling, or intercepting close-in targets using a noncoherent RF receiver to receive via a slow-wave, endfire antenna, and a microcomputer to process the received signals into guidance signals for one or more divert charge firings. An alternative embodiment receives direct fire control radar signals in addition to the reflected fire control signals for guidance processing. Alternative embodiments include piezoelectrically actuated aerodynamic augmentation in addition to or as a replacement for divert thrusters.  
         [0018]     An object of the present invention is to improve the accuracy of projectiles by an on-board guidance system with the projectile capable of sensing target-reflected, and, as an option, the direct, RF emissions of a fire control radar and the onboard guidance thereby generating trajectory-modifying commands to trajectory augmenting means. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  illustrates the general encounter geometry.  
         [0020]      FIG. 2  illustrates the orientation of the slow-wave, end-fire antenna placed longitudinally along the projectile.  
         [0021]      FIG. 3  illustrates the rotation of the projectile providing a conical scan-like mechanism for computing angle-of-arrival information.  
         [0022]      FIG. 4  is a functional block diagram of the present invention.  
         [0023]      FIG. 5  shows the electromagnetic fields of a conductor coated by a dielectric rectangular slab.  
         [0024]      FIG. 6  illustrates a corrugated conductor, which behaves physically as an artificial dielectric, comparable to that depicted in  FIG. 5 .  
         [0025]     FIGS.  7 A-B illustrate a conical scan and resulting sinusoid.  
         [0026]      FIG. 8  illustrates a typical discrimination curve.  
         [0027]      FIG. 9  illustrates the general shape of an antenna pattern of the present invention.  
         [0028]      FIG. 10  illustrates the general shape of an antenna pattern of the present invention affected by squint angle at 0 degrees rotation.  
         [0029]      FIG. 11  illustrates the general shape of an antenna pattern of the present invention affected by squint angle at 180 degrees rotation.  
         [0030]      FIG. 12  illustrates the difference formed by the antenna outputs at 0 degrees and 180 degrees.  
         [0031]      FIG. 13  illustrates the sum formed by the antenna outputs at 0 degrees and 180 degrees.  
         [0032]      FIG. 14  illustrates the discriminator as the ratio of the difference over the sum.  
         [0033]      FIG. 15  illustrates the discriminator output being proportional to angle off boresight and independent of signal amplitude.  
         [0034]      FIG. 16  illustrates the discriminant axis as rotating in space.  
         [0035]     FIGS.  17 A-D illustrate the rotation of the body-fixed polarization vector.  
         [0036]      FIG. 18  is a block diagram of the signal processing of the preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0037]      FIG. 1  illustrates the general encounter geometry  100  where the fire control radar  110  generates RF signals  120 , as reflected  130  from the target  140 , are received by a projectile  150  with a stabilizing spin  155 . As an alternative embodiment, the projectile additionally receives and processes the direct radar transmissions  160  of the fire control radar.  
         [0038]      FIG. 2  illustrates, by way of example and not limitation, the autonomous radar-directed projectile of the present invention  150  and more particularly illustrates the end-fire, slow wave antenna element  220  longitudinal orientation relative to the projectile body  210 , that is, along the length of the projectile and the location of the guidance electronics  230  at the aft portion of the projectile body  210 . The characteristic of the end-fire mechanism is that the width of the radar receive beam is determined by the length not the width of the antenna. By placing such a structure along the projectile, the present invention exploits the maximal available dimension. It is well understood by practitioners in the art of RF antennas that because of the slow wave propagation, the antenna&#39;s effective electrical length will be longer than its physical length, which will further contribute to narrowing the beam. In addition, it is well understood that the antenna beam will be squinted with respect to the boresight (i.e., axis of symmetry) of the projectile because of the physical structure and material properties of the projectile and the characteristics of the antenna.  
         [0039]     In one embodiment the radar-directed projectile possesses a stabilizing spin rate upon exit of a rifled gun muzzle and this spin will persist until target impact.  FIG. 3  illustrates that because of the projectile spin  310 , the squinted antenna beam  320  of the slow wave, end-fire antenna  220  is rotated about the boresight  330 , i.e., the principal, longitudinal, axis, of the projectile  155 . The rotating  340  of the beam  320  functions analogously to a conical scanning mechanism and thus supports an angle measurement approach similar to conical scan.  
         [0040]     Practitioners in the art will recognize that a RF source of appropriate frequency, oriented in collinear fashion with the projectile boresight  330 , will deliver a substantially constant energy on the receiving antenna element. Whereas, a RF source oriented off the boresight will cause the element to receive energy and generate voltages that appear as a modulated sine wave whose frequency will be the rotation rate of the projectile. The amplitude and phase of this sine wave is used to determine the target&#39;s position with respect to the body coordinates of the projectile.  
         [0041]      FIG. 4  shows a functional block diagram of the RDP subsystems  400  employing, for example, divert charges as a means for correcting the projectile&#39;s trajectory. The RDP, as embodied in a projectile, comprises: (a) a slow wave, end-fire antenna  220 ; (b) a non coherent receiver preferably integrated into a single GaAs chip  420  further comprising a bandpass filter  422 , a gain stage  424  and a detector  426 ; (c) a single chip microcomputer  430  comprising an analog-to-digital converter  432 , a processor  434 , memory  436 , and an input-output interface  438 ; (d) an array  440  of explosive divert charges  445 ; a video filter  450  and (e) a power subsystem that comprises a power supply and one or more batteries(not shown). Preferably, the total electronics comprised no more than three relatively small, monolithic components.  
         [0042]     In a classical conical scan system, a gimbal is typically adjusted to place the target signature in the center of the discriminant function. The body-fixed antenna element precludes gimbal adjustment approaches. In monostatic, linearly-polarized, conical scan, the return signal is independent of the angular orientation of the radar. In a system in which the antenna is body mounted, the independence is lost because as the projectile rotates, the polarization vector of its body-fixed antenna rotates. The received signal polarization is only a function of the target and the transmitter. The return signal is multiplied by the dot product of the receive antenna polarization and the polarization of the received signal. The received signal is multiplied by the rotational position of the projectile and the polarization vector of the target. Because of the nature of the polarization effect is not being a function of the position of the target off boresight, the angle discriminant function eliminates the polarization effect. In cases where the received polarization vector can change rapidly with respect to the projectile rotation rate, two antennas will be required.  
         [0043]     The preferred embodiment with its conical scan mechanism described above exhibits the economy of a single antenna. Alternative embodiments include multiple antenna elements. An alternative embodiment employing two end-fire antenna elements and two input receiver channels provides two channels of output preferably processed using an Amplitude Comparison Monopulse Algorithm (ACMA). This approach of two or more elements and an algorithm like the ACMA, while more complex, produces improved quality of angle measurements for wider antenna beams.  
         [0044]     In driving to minimal size and cost with existing technology, the receiver system of the RDP is relatively straightforward. As was illustrated in  FIG. 4 , the receiver path consists of a filter  422 , that, in the preferred embodiment is preceded by a low noise amplifier (LNA)  421 , a gain stage  424  follows the filter  422 , and precedes a noncoherent detector  426 , where, in a preferred embodiment, the noncohoerent receiver is a crystal video receiver. All of these particularly elements are preferably embodied in a single GaAs chip  420 . Such a system lacks adequate signal processing gain for most conventional radar applications. However, it is well suited for the RDP application because it can take advantage of the high illumination power of the fire control radar, whether as reflections from the target or as signals received directly.  
         [0045]     An analog-to-digital converter  432  (“A/D converter” or ADC) samples the output of the detector. The ADC is embedded preferably in an adjacent microprocessor  430  and in the preferred embodiment, the samples of the output of the detector  428  are passed through a video filter  450  before being passed to the ADC  432 . The microprocessor executes digital signal processing software that works to extract angle information using either the conical scan or an amplitude comparison monopulse algorithm. The angle modulation information is used to develop angle error signals that are fed to digital filtering, a Kalman tracking filter for example, that is also implemented as software in the microprocessor. The digital filtering will have the effect of providing an optimal, or nearly optimal, filtering time for the angle-of-arrival information.  
         [0046]     In addition to being able to extract angle information, this invention is capable of extracting range information. The fire control radar will illuminate both the target and, due to geometric proximity, the projectile. Because the RDP will know the pulse repetition frequency (PRF) of the transmitting radar, it can use this information to calibrate its internal clock; a clock that can be used for range estimation. The preferred antenna element used in this invention detects the transmitted radar pulses through its backlobe. By measuring the time between the receipt of the transmitted pulse and its identified reflection from the target, the signal processing within the microprocessor computes the range to the target.  
         [0047]     After generating a discrete steering command sequence based on determined miss distance minimizing (homing) parameters like range-to-go, the stabilized error angle equivalents of line-of-sight angle and angular velocity, discrete guidance command the RDP steers by firing at least one divert charge stored within the projectile. A more granular firing train can be achieved by well-timed and successive firings of the divert charges. An alternative guidance approach is generally continuous in implementation of the guidance law and steering commands and uses piezoelectrically actuated aerodynamic surfaces. The RDP processing tracks its target with respect to its own body reference frame. The RDP processing manages the divert firings based on a priori information regarding the position of the divert charges with respect to its own body coordinates. So long as coning angle and motion, as well as the nutation and precession motions, remain negligible, then the discrete divert thrusters position and status together with the angle-of-arrival and relative range estimates will all be used to effectively fire one or more discrete divert thrusters and by doing so steer the projectile toward an intercept with the target without the use of inertial instrumentation.  
         [0048]     Slow Wave, End-Fire Antennas  
         [0049]     The one or more antennas applied to the RDP are lightweight and mechanically simple. A slow wave structure is well suited to homing projectile application. This particular antenna launches a surface wave along the structure to its end.  
         [0050]     At least two implementations of a slow wave end-fire structure are applicable in the present invention. The first is a dielectric rod or a dielectric slab, the specific cross section of which is dependent on the projectile and fire control radar used, and the other is a corrugated metal conductor. In addition, the slow wave structure is conformal to the exterior projectile profile and, by necessity, can be embedded in the skin of the projectile.  
         [0051]      FIG. 5  shows the electromagnetic fields of a conductor coated by a dielectric rectangular slab. Propagation is in z direction. Exponential decay is in x direction. This is the dielectric slab alternative of the slow wave, end-fire antenna.  
         [0052]     The electric and magnetic fields shown in  FIG. 5  apply to either structure. Slowing the wave makes the electrical length longer and will help to increase gain and decrease the main beamwidth.  
         [0053]     The dielectric rod mechanism operates as a waveguide of a surface wave, trapped more tightly or less so depending on the value of the dielectric constant, with energy radiating out the ends of the rod. The slow wave character is produced by the dielectric constant of the material.  
         [0054]      FIG. 6  illustrates a corrugated conductor. The electromagnetic field looks as depicted in  FIG. 5  where propagation is in z direction. The corrugated metal structure of  FIG. 6  operates in a fashion similar to dielectric rod mechanism with the slow wave characteristic deriving from the metal teeth.  
         [0055]     Dielectric Constants and Slowing the Wave  
         [0056]     The two embodiments exhibit effective dielectric constants that slow the wave as it propagates along the z-axis. For the case of the corrugated conductor, the effective dielectric constant is given by a transcendental equation that relates it to the geometry of the corrugated conductor (for example the height, d, of the corrugated metal teeth) and to the various propagation constants of the artificial dielectric, that the corrugation generates.  
         [0057]     As described, the range estimation relies on receiving the transmitted pulse in the backlobes of the slow wave, end-fire antenna. A slow wave antenna, such as those applied in the present invention, possess gains in their backlobes on the order of 25 dB to 30 dB below the peak gain. Where the fire control radar is of sufficient strength, the illumination through the backlobe of the dielectric rod antennas will suffice for detection. Where fire control radar is of insufficient strength, the backlobe does not provide adequate gain to detect the transmitted pulse, and a short dipole or slot antenna (depending on the needed polarization for successful detection) can be affixed to the end of the projectile. A power splitter to the receiver connects the additional antenna. The preferable power splitting is n:1, where n is larger than 2, such that most of the rod antenna power will be channeled to the receiver.  
         [0058]     Noncoherent Receiver  
         [0059]     The electronics, as illustrated in  FIG. 4 , are preferably as simple as practicable. The receiving electronics comprises a microwave filter followed by a GaAs amplifier and a diode detector. Preferably, the amplifier and detector are on a single GaAs chip. They are followed by a microcomputer in which the ADC, memory, processor electronics are preferably all packaged on one chip as well. An example of some of this small microwave technology prior art is shown in the active microwave filter in  FIG. 18 .  
         [0060]     The use of a noncoherent detector  426  (see  FIG. 4 ) provides a good deal of simplification and with it, cost effectiveness. It reduces the required sample rate of the microcomputer ADC  432  to a multiple of the fire control radar pulse repetition interval (PRI). While the ADC  432  will take a sample every range gate between transmitted pulses, once the target has been acquired in range and then angle processing is initiated, the processor will only look in the range gates in which the target is expected to be present.  
         [0061]     Microcomputer  
         [0062]     As described, the microcomputer  430  operates at relatively low data rates because of the noncoherent video detection. The microcomputer provides the hardware functions of: receiver control; timing generation; program memory management; data memory management; analog-to-digital conversion; and providing firing strobes to the divert charges. It provides the software functions of: target detection; radar range tracking; angle-of-arrival determining; timing calibration (using received PRF of the fire control radar); digital filtering (e.g., Kalman filtering) of angle and relative range information; and computing the time to fire divert charges.  
         [0063]     Conical Scan and Discrimination  
         [0064]     FIGS.  7 A-B illustrate a conical scan and resulting sinusoid where the receptivity of the antenna is squinted off boresight producing a cycling radar return amplitude. Accordingly, the location of Maximum determines angular position of target and the extent of modulation, that is, the difference between the maximum and minimum, determines angular radial distance of target from the conscan center.  FIG. 8  illustrates a typical discrimination curve where the magnitude of the angle measurement voltage is a function of the antenna squint angle, the angle off boresight and the shape of the antenna beam.  FIG. 9  illustrates the general shape of an antenna pattern of the present invention in one dimension.  FIG. 10  illustrates the general shape of an antenna pattern of the present invention affected by squint angle at 0 degrees rotation.  FIG. 11  illustrates the general shape of an antenna pattern of the present invention affected by squint angle at 180 degrees rotation.  FIG. 12  illustrates the difference formed by the antenna outputs at 0 degrees and 180 degrees.  FIG. 13  illustrates the sum formed by the antenna outputs at 0 degrees and 180 degrees.  FIG. 14  illustrates the discriminator as the ratio of the difference over the sum.  FIG. 15  illustrates the discriminator output  1510  being proportional to angle off boresight  1520  and independent of signal amplitude. To extend from one dimension to two dimensions we can think of the discriminant axis as rotating in space. To generate azimuth and elevation components, one extracts the in-phase and quadrature components of the measured angle.  FIG. 16  illustrates the discriminant axis  1620  as rotating in space an angle, θ i , from a reference axis, where the azimuthal angle can be found from  
         [0065]     Az=ΣΨ i  cos(θ i ) and the elevation angle can be found from  
         [0066]     El=ΣΨ i  sin(θ i ).  
         [0067]     FIGS.  17 A-D illustrate the rotation of the body-fixed polarization vector and polarization effect in bistatic linearly polarized radar. As projectile  150  rotates the polarization vector  1720  of its antenna  1730  rotates. The received signal polarization is only a function of the target and the transmitter. The return signal is multiplied by the dot product of the receive antenna polarization and the polarization of the received signal. The received signal is multiplied by the rotational position of the projectile and the polarization vector of the target. The angle discriminant function eliminates the polarization effect. The polarization effect is not a function of the position of the target off-boresight. The discriminant eliminates the polarization effects.  
         [0068]     But, one must take care in using too low of signal levels. When the received polarization vector can change rapidly with respect to the projectile rotation rate, two antennas will be required as an alternative embodiment.  
         [0069]     Performance and the Preferred Embodiment  
         [0070]     System performance is a function of a number of critical parameters. Projectile effectiveness can be assessed in those critical attributes that contribute to performance. The probabilities of detecting, tracking and intercepting the target are important statistics that spring from real-world parameters. Generally, with regards to the frequency of illuminating radar, a higher frequency provides better performance.  
         [0071]     Similarly, a longer antenna element, and a bigger projectile with less ogive would end in radar performance. The Radar Cross Section (RCS) of the target and the range at which target is engaged, where closer range means more illuminating radar power, both relate back to the detection, tracking and intercepting of the target.  
         [0072]      FIG. 18  is a block diagram of the signal processing of the preferred embodiment of the present invention. The figure illustrate the Front End MMIC  1810  comprised of a low noise amplifier (LNA)  1830 , a bandpass filter  1840  and parameters  1842 , a power amplifier (PA)  1850  and a video detector  1860  and parameter  1862 . Both the LNA  1830  and PA  1850  are also illustrated by example with a chip circuitry  1832  and parameters  1834 . The preferred embodiment provides adequate acquisition range  
         [0073]     The present invention may of course be practiced in ways other than those specifically set forth herein without departing from the spirit and essential characteristics of the present invention. The present embodiments disclosed are therefore to be considered in all respects as illustrative and not restrictive or otherwise limiting and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.  
         [0074]     Alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it-should not be taken as limiting the invention as defined by the following claims.  
         [0075]     The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, materials or acts beyond the scope of the commonly defined meanings. Thus, if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself.  
         [0076]     The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result.  
         [0077]     In addition to the equivalents of the claimed elements, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements.  
         [0078]     The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptually equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention.