Abstract:
A method and apparatus for data disks adapted to compensate for effects of shock loads imparted to a disk drive which are of sufficient magnitude to cause disk shifts. A series of topographical features are formed on the disk surface or edge to cause signal fluctuation in a sensor during read-back mode operation. An initial signal profile is stored as a map indicative of track or disk alignment with respect to the actuator assembly. Selectively, such as after a known shock event, the topographical features are re-profiled. A comparison of the profiles provides a measure of any track distortion which must be compensated for in future read-write operations by adjustments to data track servo follower algorithms.

Description:
This application claims benefit of Provisional No. 60/088,335 filed Jun. 4, 1998, and Provisional No. 60/088,127 filed Jun. 4, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to rotating disk technology, particularly to disk drive apparatus employed as a memory device and, more specificaly, to a technique for determining computer hard disk alignment and alignment shifts and for providing compensation. 
     2. Description of the Related Art 
     In the state of the art of digital data storage, hard disks form the basis for main memory in most computer systems. Hard disk drives provide rapid access to a very large number of data records. Magnetic storage hard disk drives are faster than floppy disk, magnetic tape, and optical disk drives, and cost less than equivalent capacity semiconductor memory. Fundamentals of magnetic recording and disk drives can be found in  The Complete Handbook of Magnetic Recording  by Finn Jorgensen, copyright 1988 (3rd Edition, TAB BOOK Inc., Blue Ridge Summit, Pa.) 
     FIG. 1 (Prior Art) illustrates the fundamental components of a computer memory, hard disk apparatus (generally referred to simply as the “hard drive”), head-disk assembly  101 . A stack  103  of individual disks  103   a , each having magnetic recording surface layers (on both sides of the disk), rotates (arrow  105 ) at a constant speed (e.g., approximately 5400 RPM) on a hub  106  suitably mounted on a base plate  108 . Mounted in a head stack assembly  111 , load beams  107  extend magnetic transducers (“heads”)  109  which are selectively positioned with respect to each disk&#39;s recording surface in order to read and write digital data on the disks  103   a  (see also FIG.  2 ). A coil  113  on the head stack assembly  111  interacts with a magnetic drive subassembly  117  to swing the head stack assembly via a rotary shaft, or pivot bearing,  115  reversibly driven about the shaft axis, selectively moving the heads  109  from the inner diameter data tracks to the outer diameter data tracks of the recording surfaces. Appropriate electronics (not shown) are provided for controlling positioning and performing read/write functions via the heads  109 . (See also FIG. 2 regarding disk formatting). The head stack assembly  111  in combination with the magnetic drive subassembly  117  and pivot bearing  115  is referred to as the “actuator assembly.” In the state of the art, the typical read/write head may have physical dimensions of approximately 0.080-inch by 0.00125-inch; the height of the head above the disk surface may be on the order of approximately 0.75-micro-inch. Because of its shape, the head stack assembly main body part which encompasses the rotary shaft  115  is referred to as an “E-block.” A unitary E-block assembly for use in a disk drive is disclosed in U.S. Pat. No. 5,095,396 (Putnam et al., assigned to the common assignee of the present invention.) 
     FIG. 2 (Prior Art) illustrates the fundamental constructs (dimensions exaggerated) involved in recording data on a magnetic disk  203 . Physically, a hard drive disk comprises an aluminum substrate platter having a series of thin film layers thereon used in the data recording process and an outer, protective lubricant layer open to the environment (see e.g., U.S. Pat. No. Re. 32,464 (Aine) and its related patents). The disk  203  is formatted by dividing it into sectors  212  by a number of radially-extending spokes  214  placed at regular angular intervals about the disk. The spokes  214  are areas on the disk containing tracking servo bursts and sector identification information. The sectors  212  are areas on the disk containing data blocks  216 , each block having a fixed amount of data, for example 512-bytes. The data blocks  216  occupy circular tracks  218 . The tracks  218  are grouped into bands  220 ; all of the tracks  218  in a given band  220  contain the same number of radially aligned data blocks  216 . The section of a band  220  where a set of radially aligned data blocks  16  is recorded is called a block frame  222 . The number of block frames  222  per band increases with band  220  radius. The beginning and end of a block frame  222  are defined by a timing system in a disk controller (not shown). The number of bands  220  is maximized if each band has exactly one more block frame  222  than its inwardly adjacent band. In that case, the number of bands  220  is simply the difference between the number of block frames  222  in the outermost band and the number of block frames in the innermost band. The spokes  214  are numbered, e.g., zero to seven, in the direction opposite to disk rotation. The sectors  212  are also numbered, each sector being numbered the same as the immediately preceding spoke  214 . The block frames  222  in each band  220  are numbered starting from zero; block frame zero in each band is adjacent to spoke zero. In order to maximize storage capacity of the disk  210 , the data blocks  216  along the innermost track  218  of each band  220  are recorded as close as possible to a predetermined maximum linear bit density. As a result, the data rate in megabytes per second in each band increases with radius. The number of tracks  218  is predetermined for a given disk, normally spaced as close as possible to maximize storage density. In the state of the art, typical track density is approximately fifteen thousand (15,000) TPI; a typical data bit density is approximately two-hundred twenty thousand (220,000) BPI; average access time to find any particular data stream is approximately five-to nine milliseconds, holding several gigabytes of information. 
     The location of any recorded programs and data is stored in a directory area on a disk and informs the disk operating system about the exact sector and track number where the recorded data are to be found. Servo burst signals recorded in the spokes  214  provide positioning information; generally, amplitude and phase of servo-signals provide correction signals to the motor drive electronics associated with the actuator assembly. As known in the art, the design of servo-follower mechanisms, associated electronics, and optimized servo-tracking algorithms requires an analysis of the specific disk drive design implementation. While servo-tracking algorithms can provide for track following where small distortions are involved (e.g., ±100-microinch), inherent limitations in servo-tracking algorithms limits the tracking correction which is available for gross track shifts such as might occur if the head-to-track alignment is skewed, such as by a shock event. With the recognition of the advanced state of the art of such servo technology, a further detailed description is not necessary to an understanding of the present invention for a person skilled in the art. 
     As should now be recognized, considering the speed of the disk, the size of the heads, the complex data formatting on the disk, servo-follower limitations, and the high track and bit densities, for reliable read/write functionality it is critical that head-to-track alignment be precise. 
     Disk drive durability has to be maintained under sometimes severe environmental conditions, particularly during computer assembly and in the actual use of portable computers. One of the parameters that affect drive durability is shock, used to describe impact loading of drives. Shock is characterized by its magnitude in G-forces and shock duration. In disk drive technology, withstanding a shock implies that the head-media interface reliability is not compromised due to violent dynamic response of drive components following the shock event. To improve drive insensitivity to both computer assembly process, transportation, in-use handling conditions, and any other environmental situation in which a shock might be imparted to a unit, drive manufacturers are forced to design systems able to withstand higher G-forces over shorter durations; an exemplary shock load specification goal is for a disk drive to withstand a shock of 1000-G&#39;s at 1-millisecond without affecting performance. 
     One of the primary drive failure modes caused by a shock event is referred to as “disk shift” or “disk slippage.” As shown in FIGS. 3A and 3B and discussed in more detail elsewhere in this specification, following a shock event, disk shift may have changed the head-to-track alignment. Due to a gross shift of written tracks of a formatted disk, the head is unable to follow during a read cycle, resulting in data read mode failures. There is a need for a method and apparatus for facilitating the determining and compensating of disk shifts. 
     SUMMARY OF THE INVENTION 
     In a basic aspect, the present invention provides a method for profiling alignment of a rotating disk having a given radius. The method includes the steps of: providing a predetermined marker on a periphery of the rotating disk; and starting at the marker, as the disk revolves about its axis of rotation, plotting variations of proximity of the periphery of the disk between the periphery and a predetermined radial position greater than the given radius such that a plot of the variations from the marker over one revolution constitutes an initial alignment profile of the disk to the predetermined radial position. By periodically re-plotting variations of proximity of the periphery of the disk between the periphery and a predetermined radial position greater than the given radius, the re-plot of the variations from the marker over one revolution constitutes a secondary alignment profile of the disk to the predetermined radial position. The comparison of the plot to the re-plot is indicative of a change of aligniment of the rotating disk. 
     In another basic aspect, the present invention is used in a computer memory hard disk drive, having at least one disk having a given radius and a given initial axis of rotation, the drive including at least one transducer, movably mounted for reading data recorded on concentric tracks of the disk. A method for determining disk shift includes the steps of: providing a predetermined edge marker on a periphery of each rotating disk; starting at the edge marker, creating a first plot of variations of proximity of the periphery of the disk to a predetermined radial position greater than the radius as the disk revolves about the given initial axis of rotation; storing the first plot as a first reference data set; periodically, starting at the edge marker, creating a second plot as a second reference data set of variations of proximity of the periphery of the disk to the predetermined radial position greater than the radius as the disk revolves; comparing the first data set with the second data set; and determining disk shift as a function of differences between the first data set and the second data set. 
     In another basic aspect of the present invention, a computer memory hard disk drive is provided, having at least one disk having a given radius and a given initial axis of rotation, the drive including at least one selectively positionable transducer for reading data recorded on concentric tracks of the disk. A method for determining and compensating for disk shift includes the steps of: starting at a point on an outer diameter of the disk, creating a first data set indicative of variations of proximity of the periphery of the disk to a predetermined radial position greater than the radius as the disk makes a single rotation about the axis; storing the first data set; periodically, starting at the point creating a second data set of variations of proximity of the periphery of the disk to a predetermined radial position greater than the radius as the disk makes a single rotation; comparing the first data set with the second data set; determining disk shift as a function of differences between the first data set and the second data set; and if the disk shift is a function value greater than a predetermined transducer-to-track alignment tolerance, compensating transducer-to-track following by the function value. 
     Another basic aspect of the present invention is a hard disk adapted for use as a memory apparatus; the disk includes: a substrate; superjacent the substrate, at least one thin film layer associated with recording digital data in a plurality of tracks thereon; and at least one topographical anomaly located in a peripheral edge of the disk providing a marker such that proximity variations of the peripheral edge to an edge proximity detector can be plotted as a function of angular location with respect to the marker as the disk rotates passed tile edge proximity detector. 
     Another basic aspect of the present invention is a disk drive read-write head gimbal assembly mount adapted for positioning at least one read-write transducer with respect to a rotating disk, the head gimbal including: body mechanisms for positioning the read-write transducer with respect to a recording surface of the rotating disk; and mounted on the body mechanisms, detecting mechanisms for measuring proximity variations between the body mechanisms and the recording surface as the disk rotates passed the detecting mechanisms. 
     In another basic aspect, the present invention provides a computer memory disk drive system. The system includes: at least one recording disk having a plurality of concentric data tracks thereon, the disk having an outer diameter edge at a predetermined radius and an axis of rotation; at least one transducer positionable for at least reading information contained on the data tracks; a transducer mount, movably mounted adjacent to the recording disk, for selectively positioning the transducer with respect to concentric tracks on the disk; attached to the mount, at least one proximity detector for alignment of the transducer to the tracks. 
     In another basic aspect, a predetermined alignment feature is embedded into the recording surfaces of a disk. As is known in the art, a change in spacing between the head and the disk surface can be recognized as magnetic or thermal flux variations. For a magnetic disk, a sequential series of individual surface anomalies can be scribed that are recognizable by the read head are embedded in the alignment track. Thus, from a start/stop gap of the feature, a series of surface anomalies is scribed along the predetermined alignment track which will induce a pattern of such effects in the read head when in a read-back-mode and aligned to that track as the disk spins. In commercial application, each disk is implanted with the alignment track feature before incorporation into a head-disk assembly. Next, the alignment track is mapped and stored. The drive&#39;s read head is used in a read-back-mode to digitally describe an initial profile, or map, of the alignment track. A digital pattern representation of the initial profile is stored as read-only data in a memory by any known manner or proprietary data processing program. The initial profile is now available for use in various operational modes of the disk drive. When a disk shift test is called for, a current profile of the test track is made and stored using the head in a read-back-mode simply by running the read head about the alignment track, reading the predetermined feature and storing the current profile. The two stored maps, the initial profile and the current profile are compared. If the maps are identical, or within a predetermined tolerance defined for a specific implementation knowing the drive&#39;s particular servo-tracking algorithm limitations, the system standard read-write mode can be initiated. If it is determined that significant track distortion has occurred due to disk shift, the servo-tracking algorithm is modified to conform track following in all future read-write mode operations. The servo-tracking modification is simple as the distortion is a direct function of track angular location to the initial configuration. Thus, the track position compensation value is a constant. The data processing involved in mapping, storing, and comparing alignment track profiles can be implemented in either a firmware and semiconductor memory form or in software as part of a disk drive&#39;s software driver program. 
     In a further basic aspect, the present invention provides a method for determining a shift of a disk having a plurality of tracks thereon. The method includes the steps of: forming a series of fixed anomalies on a surface of the disk; forming an initial profile of track alignment based on a positional profile of the series of fixed anomalies; storing the initial profile; periodically forming a current profile of the series of fixed anomalies; comparing the initial profile to the current profile; and deriving a measurement of disk shift from detected differences between the initial profile and the current profile. 
     In a further basic aspect, the present invention provides a method for determining and compensating for disk slippage of a computer memory disk having a plurality of data tracks thereon in a disk drive having a read head adapted for following the data tracks. The method includes the steps of: forming a series of fixed anomalies on a surface of the disk; forming an initial profile of data track alignment from the series of fixed anomalies; periodically forming a current profile of data track alignment from the series of fixed anomalies; comparing the initial profile to the current profile; deriving a measurement of disk shift from detected differences between the initial profile and the current profile; and adjusting data track following in conformance with the measurement of disk shift. 
     In a further basic aspect, the present invention provides a track alignment pattern for a disk shaped data recording medium having at least one recording surface thereon, the surface adapted for having a plurality of concentric recording tracks thereon, the pattern comprising a sequence of geometric features scribed onto the recording surface of the disk in at least one predetermined track at a substantially constant radial distance from the axis of rotation of the disk, forming a pattern for mapping concentric alignment of the disk with respect to the axis of rotation. 
     In a further basic aspect, the present invention provides a computer memory disk adapted for use with a read head, the disk including: a substrate; superjacent the substrate, at least one thin film layer associated with recording digital data therein; and a series of anomalies in the thin film layer such that the anomalies are identified by the read head as an alignment feature for the disk. 
     In a further basic aspect, the present invention provides a disk drive including: at least one recording disk having a plurality of concentric data tracks thereon; at least one transducer for at least reading information contained on the data tracks; at least one of the data tracks being a predetermined track having a set of topographical features embedded thereon such that the transducer is usable as a mechanism for forming an initial profile of the predetermined track having a set of topographical features embedded thereon wherein the initial profile is an initial track alignment map of the plurality of concentric tracks; and mechanisms for selectively forming a current profile of the predetermined track having a set of topographical features embedded thereon and for comparing the current profile to the initial profile and for resetting data track following of the transducer to compensate for differences between the initial profile and the current profile. 
     It is an advantage of the present invention that it provides a method for accurately determining and compensating for disk shifts. 
     It is another advantage of the present invention that relatively gross disk slippage can be accommodated with little impact to the read/write performance of the drive. 
     It is another advantage of the present invention that it can be implemented with no changes to either the heads or the fundamental media design. 
     It is another advantage of the present invention that it provides a disk drive that can withstand higher shocks without affecting performance. 
     It is still another advantage of the present invention that it overcomes limitations in servo-tracking algorithms, ability to compensate for data track shifts. 
     It is a further advantage of the present invention that it provides a mechanism for determining shock event disk shift effects without requiring contact with the disks. 
     It is a further advantage of the present invention that it requires no added hardware to the disk drive construct. 
     The foregoing summary and list of advantages is not intended by the inventors to be an inclusive list of all the aspects, objects, advantages and features of the present invention nor should any limitation on the scope of the invention be implied therefrom. This Summary is provided in accordance with the mandate of 37 C.F.R. 1.73 and M.P.E.P. 608.01(d) merely to apprize the public, and more especially those interested in the particular art to which the invention relates, of the nature of the invention in order to be of assistance in aiding ready understanding of the patent in future searches. Other objects, features and advantages of the present invention will become apparent upon consideration of the following explanation and the accompanying drawings, in which like reference designations represent like features throughout the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 (PRIOR ART) is an illustration of a computer memory hard disk drive actuator assembly. 
     FIG. 2 (PRIOR ART) is a schematic representation of a computer memory disk, showing formatting thereon, as used in a head stack assembly of the actuator assembly of FIG.  1 . 
     FIGS. 3A and 3B are graphical demonstrations of the problem of head-track misalignment. 
     FIG. 4 is a schematic illustration, elevation view, of a first embodiment of the present invention. 
     FIG. 5 is a schematic illustration, elevation view, of a second embodiment of the present invention. 
     FIG. 6 is a flow chart of the methodology for determining and compensating for data track profile distortion due to disk shift in accordance with the present invention as shown in FIGS. 4 and 5. 
     FIG. 7 is an exemplary disk profile based on edge proximity sensing in accordance with the present invention as shown in FIGS. 4,  5  and  6 . 
     FIG. 8 is a flow chart of the methodology for determining and compensating for data track profile distortion due to disk shift in accordance with another embodiment of the present invention. 
     FIGS. 9A and 9B are a schematic representation of a geometric feature of a disk in accordance with the present invention as set forth in FIG. 8 in which: 
     FIG. 9A shows a before slippage mapping, and 
     FIG. 9B shows an after slippage mapping thereof. 
    
    
     The drawings referred to in this specification should be understood as not being drawn to scale except if specifically noted. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference is made now in detail to a specific embodiment of the present invention, which illustrates the best mode presently contemplated by the inventors for practicing the invention. Alternative embodiments are also briefly described as applicable. 
     Turning to FIG. 4, a simplified schematic representation of the present invention is shown. An E-block  401  includes a plurality of cantilevers  403 . Read-write heads, or slider,  405  are moulited to the E-block cantilevers  403  on extension arms suspensions  407 ; in combination, the suspensions and sliders are also referred to in the art as a “load beam assembly.” The load beam assembly  403 ,  407  essentially provides the mechanical interface for flying the heads  405  across the recording surfaces of each disk  409  while the disk is rotating about the axis of rotation S - - - S (“disk spin”). A disk edge, or “outer diameter,” sensor  411  for each disk  409  of the stack  409 ′ is mounted on the cantilevers  403 . A suitable, commercially available, optical sensor can be employed in accordance with the present invention. Other types of proximity and alignment sensitive devices known in the art, such as capacitance probes, can also be employed in accordance with the present invention. With optical sensors  411 , a beam of light  413  is projected onto the edge  415  of each disk  409  and a return beam received back by a photodetector cell of the sensor, measuring instantaneous proximity of the disk edge  415  to the sensor  411 . In order to be useful for head-to-track alignment, it is appropriate to position the transducer head  405  over a known, reference track when creating a plot of the edge  415  proximity to the fixed position sensor  411 ; this is discussed further with respect to FIG. 5 below. 
     Referring also to FIG. 6, when a disk drive is built or following assembly into a computer system, the sensors  411  adjacent respective disks  409  of the stack are used to create an initial alignment profile(s) which is digitized and stored in a memory based on the data produced by the sensor as a disk rotates passed, step  601 . 
     An exemplary plot of the data is shown in FIG. 7 where the distance between the sensor  411  and the periphery of the disk varies with angular rotation when the read-write head is positioned over a predetermined edge-referencing track. The disk edge  415  is provided with a marker, such as a dimple having a depth relatively greater than any expected edge proximity deviations, to provide a signal indicative of a complete revolution, as shown in the plot at zero-degrees and 360-degrees. Any other known-manner marker can be employed. 
     During a drive boot-up  602  or following a known shock event  603 , a disk slip test  604  is performed. A current profile for each disk is generated and stored  605  in a like manner as when creating the initial disk alignment profiles as explained above. The initial profile and the current profile can then be compared  606 . When the initial profile and the current profile are substantially the same, namely when no slippage is found which would affect the head-to-track alignment based on the design criterion of the specific disk drive apparatus, the test is ended and the drive controller returned to the read-write mode  608 . If a tolerance for proper head-to-track alignment has been exceeded in discovered differences between the initial and current profiles, data track distortion is plotted  609  as a function of angular rotation of the disk. The head-to-track drive servo algorithm of the specific disk drive apparatus is modified  610  accordingly to allow proper track following in future read-write operations. 
     FIG. 5 is another embodiment of the present invention where the sensors  411  are mounted to the E-block journal bearing  501  rather than on the cantilevers  403 . But note that the present invention can also be used in a linear motion head actuator assembly rather than the rotary actuator shown. In a rotary actuator, as the sensors will move with the E-block, depending on the specific implementation and the amount of relative radial swing motion of the cantilevers, it may be appropriate to create the plots of disk edge proximity while the transducers are parked over a particular selected track, such as on or near a mid-disk track, as radial E-block motion during a test could affect readings. Plots subsequent to the initial plot should be made with the E-block in the same rotational position as when the initial plot readings were taken. 
     Referring now to FIGS. 8,  9 A and  9 B, a predetermined alignment feature  901  is embedded into the recording surfaces of a disk  903 , step  801 . FIG. 8 is similar in content to FIG. 6, but is included to facilitate description of further features and embodiments of the present invention. 
     Continuing the example of the just mentioned figures, a topographical geometric feature is formed on a predetermined track to be used for track alignment sensing using a commercial laser, shown herein as being near the disk  903  inner diameter  905 . Any track or tracks may be selected for use as the alignment track with the predetermined alignment feature  901  as suits a particular implementation. As is known in the art, a change in spacing between the head and the disk surface can be recognized as magnetic or thermal flux variations using a magneto-resistive (“MR”) head. For a magnetic disk, a sequential series of individual surface anomalies can be scribed that are recognizable by the read head are embedded in the alignment track. 
     Thus, from a start/stop gap  907  of the feature  901 , a series of surface anomalies is scribed along the predetermined alignment track which will induce a pattern of such effects in the read head when in a read-back-mode and aligned to that track as the disk  903  spins. While a variety of anomalies can be developed through testing, in the preferred embodiment a series of raised bumps that decrease the head-to-disk spacing by approximately fifty percent are scribed every 0.05 degrees about the alignment track. Note that in an alternative embodiment, these “anomalies” can also be continuous bump, which is concentric on the disk, or a like implementation expedient design. In commercial applications, each disk can be implanted with the alignment track feature before incorporation into a head-disk assembly. 
     Next, the alignment track is mapped and stored, step  803 . The drive&#39;s read head is used in a read-back-mode to digitally describe an initial profile, or map,  909  of the alignment track. A digital pattern representation of the initial profile is stored as read-only data in a memory, such as an EPROM chip (not shown), by any known manner or proprietary data processing program. 
     The initial profile  909  is now available for use in various operational modes of the disk drive. Particularly in portable computers, it is advisable to run a disk shift test, step  805 , on every boot-up  807  of the computer due to interim handling between usage. In programming computer boot routines, the disk shift test  805  can be auto-initialized or user-initialized; in an automated “wake-up” routine following a mere idle mode of operation, a drive is simply routed to its standard read-write mode, step  811 . Following a known shock event  809 , such as if a computer is severely jarred or dropped, a disk shift test  805  should be run even if the computer is already in the read-write mode. A simple, known manner, user initiated test program can be provided as part of the disk operating system. 
     When a disk shift test  805  is called for, a current profile  919 , FIG. 9B, of the test track  907  is made and stored using the head in a read-back-mode, step  813 , simply by running the read head about the alignment track, reading the predetermined feature  901 , and storing the current profile. The two stored maps, the initial profile  909  and the current profile  919 , are compared, step  815 . If the maps are identical, or within a predetermined tolerance defined for a specific implementation knowing the drive&#39;s particular servo-tracking algorithm limitations, step  817 , the system standard read-write mode can be initiated, step  811 . If it is determined  819  that significant track distortion has occurred due to disk shift, FIG. 9B, the servo-tracking algorithm is modified  821  to conform track following in all future read-write mode operations  811 . The servo-tracking tracking modification is simple as the distortion is a direct function of track angular location to the initial configuration. Thus, the track position compensation value is a constant. 
     It should be recognized by a person skilled in the art that the anomalies intentionally provided in the predetermined alignment track or tracks can be of any type that is detectable by the read head. These topographical anomalies can be conformed to different technologies, magnetic, thermal, optical, or the like as may be available in the state of the art. It should also be recognized by a person skilled in the art that the data processing involved in mapping, storing, and comparing alignment track profiles can be implemented in either a firmware and semiconductor memory form or in software as part of a disk drive&#39;s software driver program. 
     The foregoing description of the preferred embodiment of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form or to exemplary embodiments disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. Similarly, any process steps described might be interchangeable with other steps in order to achieve the same result. The embodiments were chosen and described in order to best explain the principles of the invention and its best mode practical application, thereby to enable others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use or implementation contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents. Reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather means “one or more.” Moreover, no element, component, nor method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the following claims. No claim element herein is to be construed under the provisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for . . . .”