Patent Publication Number: US-2007114989-A1

Title: Apparatus and Methods for Magnetic Through-Skin Sensing

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This patent application is a continuation-in-part application of co-pending, commonly-owned U.S. patent application Ser. No. 10/656,595 entitled “Apparatus and Methods for Magnetic Through-Skin Sensing” filed on Sep. 5, 2003, which application is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION  
      The field of the present disclosure relates to apparatus and methods for magnetic through-skin sensing, and more specifically, to manufacturing operations employing magnetic sensing for position location on a workpiece.  
     BACKGROUND OF THE INVENTION  
      Manufacturing operations in many fields typically require accurate positioning of manufacturing tools over a workpiece. One example is the drilling of fastener holes in the field of aircraft manufacturing. Installing fastener holes in airplane parts, particularly the panels or “skins” of an aircraft, commonly requires either “blind” drilling from an external location, or “back” drilling from within the aircraft fuselage. In either case, it may be difficult to drill in the correct location. The difficulty may be caused by the fact that the best drilling in numerous situations is done from the outside in, but the best location information about where to drill is determined by conditions on the inside (i.e. non-drilling) side of the part.  
      One conventional approach to this problem is to drill a reduced diameter pilot hole from the inside out, and then complete the hole from the outside in, guided by the pilot hole. Another compromise approach is to transfer the location from the inside to the outside using a mechanical guide or measurement device.  
      Although desirable results have been achieved using the prior art drilling systems and methods, there is still room for improvement. The above-referenced prior art methods may be time and labor intensive, thereby reducing the efficiency of the manufacturing operations. Therefore, a need exists for improved positioning systems and methods for performing manufacturing operations on a workpiece.  
     SUMMARY  
      The present invention is directed to apparatus and methods for magnetic through-skin sensing, and more specifically, to manufacturing operations employing magnetic sensing for position location on a workpiece. Apparatus and methods in accordance with the present invention may advantageously improve the efficiency, throughput, and accuracy of manufacturing operations on a workpiece.  
      In one embodiment, a sensing system includes a first portion including a magnet having, and a second portion including a magnetic field sensor. The magnet has a magnetic field emanating therefrom. A field-directing member provides a zone of approximately spherically-shaped magnetic force gradient lines, the zone having an angular extent of at least approximately three degrees and the zone extending through the workpiece. The magnetic field sensor is moveable through at least a portion of the zone. The magnetic field sensor senses a characteristic of the shaped magnetic field portion indicative of the desired position for the manufacturing operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Embodiments of the present invention are described in detail below with reference to the following drawings.  
       FIG. 1  is a side elevational view of a sensing system in accordance with an embodiment of the invention;  
       FIG. 2  is a side elevational view of a sensing system in accordance with an alternate embodiment of the invention;  
       FIG. 3  is a side elevational view of a sensing system in accordance with another alternate embodiment of the invention;  
       FIG. 4  is an isometric view of a representative manufacturing assembly in accordance with yet another embodiment of the invention;  
       FIG. 5  is a flowchart of a method of manufacturing incorporating a method of sensing in accordance with a further embodiment of the invention; and  
       FIG. 6  shows a representative magnetic field for an apparatus in accordance with an embodiment of the invention.  
    
    
     DETAILED DESCRIPTION  
      The present invention relates to apparatus and methods for magnetic through-skin sensing, and more specifically, to manufacturing operations employing magnetic sensing for position location on a workpiece. Many specific details of certain embodiments of the invention are set forth in the following description and in  FIGS. 1-6  to provide a thorough understanding of such embodiments. One skilled in the art, however, will understand that the present invention may have additional embodiments, or that the present invention may be practiced without several of the details described in the following description.  
       FIG. 1  is a side elevational view of a sensing system  100  in accordance with an embodiment of the present invention. In this embodiment, the sensing system  100  includes a magnet  110  having a field-directing polepiece  112 . A plurality of magnetic field lines (lines of constant strength magnetic force) emanate from the magnet  110 . The magnetic field lines exhibit areas (or lines) of approximately constant magnetic force gradient, designated in  FIG. 1  as magnetic force gradient lines  114 . The field-directing polepiece  112  is suitably shaped so that at least some of the magnetic force gradient lines  114  proximate the field-directing polepiece  112  form a shaped magnetic field portion  116 . The shape and permeability of the field-directing polepiece  112  may be adjusted to control the shape of the magnetic force gradient lines  114 , and may be determined using, for example, analytical simulations, experimentation, or other suitable techniques.  
      In one particular embodiment, for example, the field-directing polepiece  112  may be an approximately conical shape, and the zone (or area or region)  117  wherein the magnetic force gradient lines  114  are of approximately spherical (including partially spherical) shape is configured as shown in  FIG. 1 . For purposes of comparison, an ideally-spherical shape  118  is superimposed on the shaped magnetic field portion  116  of  FIG. 1 , which demonstrates a relative extent of the zone of approximately spherical shape  117  relative to the ideally-spherical shape  118 . In the embodiment shown in  FIG. 1 , the zone  117  extends from a first boundary of approximately 15 degrees from horizontal to a second boundary of approximately 45 degrees from horizontal.  
      Controllably varying the shape and permeability of the polepiece  112  enables the size of the approximately spherically-shaped zone  117  to be controlled. In some embodiments, the shape and permeability of the polepiece  112  may be adjusted to maximize the size of the zone  117  in which the magnetic force gradient lines  114  are approximately spherically shaped. In other embodiments, the zone  117  may be adjusted to any other desired sizes.  
      While it may be appreciated that, in general, magnetic force gradient lines normally emanating from a pole magnet (other than a point magnetic source) that are not shaped using a field-directing polepiece may be approximately elliptically shaped, and may thus be considered to have an approximately spherical shape within a relatively small, relatively localized region having a relatively small angular extent (e.g. approximately two degrees or less), the zone  117  has an appreciably greater angular extent than such relatively localized regions produced by such non-shaped magnetic fields. In contrast, various embodiments of the present invention may be configured to operate with a zone  117  of approximately spherically shaped magnetic force gradient lines having an angular extent (or subtended angle) of approximate three degrees or greater. In other particular embodiments, the zone  117  created using the field-directing polepiece  112  may have an angular extent of approximately 5 degrees or greater, and in further embodiments, a zone having an angular extent of approximately 10 degrees or greater may be employed.  
      For example,  FIG. 6  shows a representative magnetic field  600  for a particular configuration of a magnet  602  and a polepiece  604  in accordance with another embodiment of the invention. In this embodiment, the polepiece  604  is sized and positioned with respect to the magnet  602  to provide a zone  617  in which the magnetic force gradient lines  606  are approximately spherically shaped. More specifically, in this embodiment, the zone  617  extends from a first angle α 1  of approximately 5 degrees to a second angle α 2  of approximately 55 degrees (i.e. a subtended angle of approximately 50 degrees). In an alternate embodiment, the properties of the polepiece  604  may be adjusted to provide a region  627  of approximately spherically shaped magnetic force gradient lines having an angular extent (or subtended angle) Δα of at least approximately three degrees (or greater).  
      In the embodiment shown in  FIG. 6 , the polepiece  604  has an approximately conical shape with a height of approximately 0.75 inches, a base diameter of approximately 0.5 inches, and a magnetic relative permeability of approximately 50,000. In addition, in the configuration shown in  FIG. 6 , the magnetic permeability of the polepiece  604  was assumed to be constant throughout the polepiece  604 . Of course, in alternate embodiments, any suitable polepiece shapes, permeability values, and permeability distributions may be used to achieve the desired characteristics of the magnetic force gradient lines  606 .  
      Furthermore, it will be appreciated that various manufacturing environments may require varying degrees of precision, with some manufacturing environments requiring a relatively higher degree of accuracy, and others requiring less accuracy. In general, the accuracy of various embodiments may be adjustably controlled by controlling the shape and permeability properties of the polepiece  604  to adjust the approximate sphericity of the magnetic force gradient lines  606 , and thus, the ability of the system to locate an imaginary centerpoint of the zone  617  of approximately spherical magnetic force gradient lines  606 . For example, in some manufacturing environments, such as those involving automated drilling operations on contoured or non-contoured surfaces, the magnetic force gradient lines  606  may be controlled to an approximately spherical shape such that a position of an imaginary centerpoint of the approximately spherical zone  617  may be determined to a desired tolerance of approximately +/−0.001 inches, as described more fully below. Of course, in alternate embodiments, the tolerances associated with the location of the centerpoint may be relaxed (e.g. 0.01 inches, 0.1 inches, etc.) or tightened (e.g. 0.0005 inches, 0.0001 inches, etc.) depending on the requirements of the particular application.  
      Referring again to  FIG. 1 , the sensing system  100  also includes a magnetic field sensor  120 . The magnetic field sensor  120  may be a conventional magnetic field sensor, including, for example, those sensors commercially-available as model PK 88782 industrial sensor from Honeywell International, Inc. of Morristown, N.J., model MLX90215 from Melexis Microelectronic Systems, Inc. of Concord, N.H., the 1321 series sensors from Allegro Microsystems, Inc. of Worcester, Mass., or any other suitable magnetic field sensor. In a particular embodiment, for example, the magnetic field sensor  120  may be a linear Hall effect sensor.  
      In operation, the magnet  110  may be operatively positioned relative to a workpiece  130 . More specifically, the field-directing polepiece  112  of the magnet  110  may be positioned proximate a first surface  132  of the workpiece  130  so that at least a portion of the zone of approximately spherical shape  117  extends through the workpiece  130 , and further extends outwardly beyond a second surface  134  of the workpiece  130 . In a particular embodiment, the field-directing polepiece  112  may be engaged against (i.e. in contact with) the first surface  132  at a first location  136 . Alternately, the field-directing polepiece  112  may be spaced apart from the first surface  132  at the first location  136 .  
      With continued reference to  FIG. 1 , with the field-directing polepiece  112  operatively positioned relative to the desired first location  136 , the magnetic field sensor  120  may be moved along one or more traversing paths  122  that pass at least partially through the shaped magnetic field portion  116 . In the embodiment shown in  FIG. 1 , the traversing path  122  extends at least partially through the zone of approximately spherical shape  117 . As the magnetic field sensor  120  is moved from a first sensor position  124  outside of the zone of approximately spherical shape  117  to a second sensor position  126  (depicted in phantom lines) within the zone of approximately spherical shape  117  (or vice versa from the second sensor position  126  to the first sensor position  124 ), the magnetic field sensor  120  senses the magnetic field lines  114  within the zone of approximately spherical shape  117 .  
      From the known shape of the zone of approximately spherical shape  117 , and the magnetic field strength values determined by the magnetic field sensor  120  along the traversing path  122 , a second location  140  on the second surface  134  of the workpiece  130  may be determined. In an exemplary embodiment, the second location  140  is directly opposite from the first location  136 , and the second location  140  represents a location on the second surface  134  of the workpiece  130  at which a manufacturing operation (e.g. drilling) is desired to be performed. Alternately, the second location  140  may be offset (e.g. by a predefined offset distance along the second surface  134 ) from a desired location at which a manufacturing operation is to be performed.  
      The magnetic sensor  120  may transmit one or more sensing signals to a data system  150  via a communication link  152  (e.g. an electrically conductive lead or a wireless link). The data system  150  may, in turn, process the sensing signals to determine the second location  140 , or may take other action in response to the sensing signals, including, for example, transmitting one or more control signals to operatively position a manufacturing tool for performing manufacturing operations of the workpiece  130  at a desired location, as described more fully below.  
      In one particular aspect of the embodiment shown in  FIG. 1 , the shaped magnetic field portion  116  may be suitably shaped so that the second location  140  is at an approximate center of the zone of approximately spherical shape  117 . Thus, the second location  140  (i.e. the center of the zone of approximately spherical shape  117 ) may be suitably indexed to a desired location on the second surface  134  (e.g. to mark a desired drilling location) by proper positioning of the magnet  110  relative to the first surface  132 . Furthermore, since the magnetic force gradient lines  114  of the shaped magnetic field portion  116  approximate a zone of approximately spherical shape  117  in the vicinity of the second surface  134  where the magnetic field sensor  120  is traversed, the sensing system  100  may advantageously be relatively insensitive to the normality of a longitudinal axis  111  of the magnet  110  relative to the workpiece  130 , as well as the approach angle of the magnetic field sensor  120  along the traversing path  122  relative to the second surface  136 . In one particular embodiment, for example, where the shape of the entirety shaped magnetic field portion  116  is perfectly spherical, the sensing system  100  may be extremely insensitive to such normality and approach angle conditions. Typically, in the embodiment shown in  FIG. 1 , the degree of non-normality tolerated by the sensing system  100  may be a function of the accuracy with which a sphere is approximated by the zone of approximately spherical shape  117  of the shaped magnetic field portion  116 .  
      Embodiments of sensing systems in accordance with the present invention may be used, for example, in applications where location information needs to be transferred through an opaque surface. For example, in one exemplary application, a sensing system may be employed in a process of joining an aircraft skin to structural components wherein the fastener locations may be determined from the inside of the structure via predrilled pilot holes, and the location of those pilot holes needs to be determined from the outside of the applied skin. In such an application, the magnet and the field-directing polepiece would typically be located on the interior side of the aircraft skin adjacent the pilot hole, and the magnetic field sensor would be traversed along the outside of the aircraft skin, where there is no visible means of locating the pilot hole.  
      Alternately, in further embodiments, sensing apparatus and methods in accordance with the present invention may be used in any number of different drilling applications where a location is known on a back side of a workpiece and the information needs to be transferred to a front side in order to properly locate the hole. It may also be appreciated that embodiments of sensing apparatus and methods in accordance with the invention are not restricted to drilling applications, but rather, may generally be used for transferring location information through an opaque medium, preferably a non-magnetic medium. Such alternate applications include, but are not limited to, orienting laminate countertop in carpentry, wood veneers in musical instrument making, fabric coverings in modular office furniture, and an unlimited number of other applications requiring transfer of positional information through an opaque material.  
      The sensing system  100  may provide significant advantages over alternate, prior art systems. For example, because the magnetic field sensor  120  may be swept along any a wide range of desired traversing paths  122  to determine the second location  140 , the sensing system  100  may be easier and more efficient to operate in comparison with alternate systems. Also, as noted above, the sensing system  100  may be relatively less sensitive to non-normality conditions in comparison with prior art systems, thereby providing improved accuracy. Overall, the sensing system  100  may advantageously provide a cost-effective and accurate method of indexing a desired location for performing a manufacturing operation on an outer surface of a workpiece from an inner surface of the workpiece.  
      It will be appreciated that the workpiece  130  may be a substantially planar workpiece as shown in  FIG. 1 , or alternately, may be any other suitably contoured or non-planar shape. Preferably, the workpiece  130  may be a non-ferromagnetic material so that the shaped magnetic field portion  116  is not distorted, attenuated, or otherwise degraded from its desired shape, strength, or other quantitative or qualitative property. Alternately, if the workpiece  130  includes a ferromagnetic material that may adversely impact the shaped magnetic field portion  116 , suitable empirical calibrations or other suitable adjustments may be necessary in order to account for such adverse effects so that the second location  140  may be determined with acceptable accuracy.  
      Furthermore, it will be appreciated that the ambient magnetic environment surrounding the sensing system  100  should preferably be substantially less than the strength of the shaped magnetic field portion  116 . Furthermore, it is desirable that the ferromagnetism of any structure(s) in the vicinity of the sensing system  100  may be negligible, or alternately, approximately homogeneous so as not to appreciably distort the shape of the shaped magnetic field portion  116 , especially the zone of approximately spherical shape  117 . If, however, instances of repeatable inhomogeneity exist, then the shape of the field-directing polepiece  112  may be appropriately modified to accommodate these instances.  
      It may be noted that the field-directing polepiece  112  of the magnet  110  is not limited to the particular shape shown in  FIG. 1 , and may be configured in a wide variety of alternate shapes which may in turn produce other shaped distributions of magnetic flux. For example, in an alternate embodiment, a field-directing polepiece that includes a cylindrical portion with a cone cut-away portion (or frustrum) from the longitudinal axis  111  may produce a more cylindrically-shaped magnetic field portion, and a cylindrical polepiece may produce a more elliptically-shaped magnetic field portion. It may also be appreciated that the shape of the magnetic field portion proximate the field-directing polepiece may be modified in ways other than by the external shape of the field-directing polepiece. For example, in another alternate embodiment, the field-directing polepiece may include an insert portion having a different magnetic permeability than an outer portion of the field-directing polepiece so that a shape of the shaped magnetic field portion may be further modified. In yet another embodiment, successive annular rings of differing magnetic permeability materials (e.g. decreasing permeability for successively smaller rings) may be nested such that the flux density may be forced to inhabit areas preferentially over lower total permeability, thus allowing the overall magnetic flux shape to be varied as desired.  
      Thus, although the above-described embodiment of the shaped magnetic field portion  116  includes the zone of approximately spherical shape  117  in the vicinity of the second surface  134 , a variety of alternate embodiments are possible. In some alternate embodiments, particularly those embodiments having an axially symmetrical magnetic field portion (such as an approximated cylinder or ellipsoid shape), it may be desirable and advantageous to restrict the movement of the magnetic sensor  120  along the traverse path  122  to lie within a particular plane of motion in order to uniquely define a center of the shaped magnetic field portion, thereby providing a more accurate determination of the second location  140  on the workpiece  130 .  
      In the following discussion, several alternate embodiments of apparatus and methods are described with reference to  FIGS. 2-5 . Throughout these discussions, not all of the details of the structural and operational aspects of these additional embodiments will be repeated from the description provided above, but rather, for the sake of brevity, only the more significant aspects and differences of the structural and operational characteristics of such alternate embodiments will be described.  
       FIG. 2  is a side elevational view of a sensing system  200  in accordance with an alternate embodiment of the invention. In this embodiment, the sensing system  200  includes a magnet  210  that emanates a plurality of magnetic force gradient lines  214 , and having a field-directing polepiece  212  that provides a shaped magnetic field portion  216  that includes a zone of approximately cylindrical shape  217 . For comparison, an ideally-cylindrical shape  218  is superimposed on the shaped magnetic field portion  216 .  
      As further shown in  FIG. 2 , in this embodiment, the field-directing polepiece  212  may include an outer portion  211  and an inner portion  213 . In one embodiment, the outer portion  211  includes a first material having a first magnetic permeability, and the inner portion  213  includes a second material having a second magnetic permeability. In an alternate embodiment, the inner portion  213  may be a hollow cutout containing air or other suitable substance, or vacuum.  
      In operation, the magnet  210  is operatively positioned relative to the first surface  132  of the workpiece  130  so that at least a portion of the zone of approximately cylindrical shape  217  extends beyond the second surface  234  of the workpiece  130 . In this embodiment, the field-directing polepiece  212  is positioned at a standoff distance  135  from the first location  136  on the workpiece  130 . The magnetic field sensor  120  is then moved along a traversing path  222  that passes at least partially through the shaped magnetic field portion  216  from the first sensor position  224  outside the shaped magnetic field portion  216  to the second sensor position  226  within the zone of approximately cylindrical shape  217  (or vice versa). In this embodiment, the traverse path  222  is restricted to lie within a plane that is a constant distance  223  from the second surface  134  of the workpiece  130 . As the magnetic field sensor  120  is moved along the traverse path  222 , it senses the magnetic force gradient lines  214  and transmits appropriate signals to the data system  150 , as described more fully above. From the known shape of the zone of approximately cylindrical shape  217 , and the magnetic field strength values determined by the magnetic field sensor  120  along the traversing path  222 , the second location  140  on the workpiece  130  may be determined.  
       FIG. 3  is a side elevational view of a sensing system  300  in accordance with another alternate embodiment of the invention. In this alternate embodiment, the sensing system  300  includes a field-directing polepiece  312  and an electromagnet  360  aligned along a longitudinal axis  311  and emanating a plurality of magnetic force gradient lines  314 . A source  362  (e.g. a current supply) is operatively coupled to the electromagnet  360 . The field-directing polepiece  312  is positioned at a standoff distance  135  from the first location  136  on the workpiece  130  and provides a shaped magnetic field portion  316  that includes a zone of field sensing  317 . In a particular embodiment, for example, the zone of field sensing  317  may be an approximately cylindrically-shaped zone.  
      In operation, the magnetic field sensor  120  is moved along a traversing path  322  extending from a first sensor position  324  to a second sensor position  326 , the first and second sensor positions  324 ,  326  being disposed within the zone of field sensing  317 . The traverse path  322  is restricted to be a constant distance  323  from the second surface  134  of the workpiece  130 . As the magnetic field sensor  120  moves along the traverse path  322 , it senses the magnetic force gradient lines  314  and transmits appropriate signals to the data system  150 , as described more fully above.  
      Also, because the sensing system  300  includes the electromagnet  360 , the overall flux density within the shaped magnetic field portion  316  may be varied (i.e. increased or decreased) by adjustment of the source  362 . In one embodiment, for example, the overall flux density may be varied to take advantage of magnetic saturation effects in portions of the core of the electromagnet  360  (or of the field-directing polepiece  312 ) to further exploit the core&#39;s effect on the shaping of the shaped magnetic field portion  316 . This may be achievable, for example, by virtue of an effect whereby field fringing may increase as the saturation does not allow the internal density to increase in the area that becomes saturated. Alternately, in further embodiments, the overall flux density may be varied to account for other factors, including but not limited to, attenuations due to varying thicknesses or varying properties of the workpiece  130 , inhomogeneities in the ambient magnetic field of the surrounding environment, or any other suitable factors. Based on the known magnetic flux density of the zone of field sensing  317 , and the measured data from the magnetic field sensor  120  along the traversing path  322 , the second location  140  on the workpiece  130  may be determined.  
      With continued reference to  FIG. 3 , in another alternate embodiment, the sensing system  300  may further include a secondary polepiece  390  operatively positioned relative to shaped magnetic field portion  316  and the second surface  134  of the workpiece  130 . The secondary polepiece  390  emanates a plurality of secondary magnetic field lines  394  which may be adapted to cause the magnetic force gradient lines  314  of the shaped magnetic field portion  316  to become relatively more concentrated in at least part of the shaped magnetic field portion  316 , thus enabling measurements to be made on a weaker field (e.g. using a weaker magnet). In the embodiment having the secondary polepiece  390 , compensation may be made to account for distortions and inhomogeneities introduced by the secondary polepiece  390 , which may be accomplished, for example, using an iterative approach, an experimental approach, a semi-empirical approach, or any other suitable compensation technique.  
      It will be appreciated that various embodiments of sensing systems in accordance with the present invention may be utilized in a stand-alone manner, or may be incorporated into a wide variety of existing manufacturing apparatus. Indeed, a virtually limitless number of manufacturing assemblies may be conceived for positioning the field-directing polepiece of the sensing system, and for traversing the magnetic field sensor, in accordance with alternate embodiments of the present invention. Such systems may range from automated, computer controlled manufacturing apparatus, to relatively-simple manually-operated apparatus, and even to simple manual activities performed by an operator. Representative manufacturing assemblies which may be utilized to perform the positioning and traversing operations involved in the operation of the sensing systems in accordance with the present invention include, but are not limited to, those manufacturing assemblies generally described in U.S. Pat. No. 4,850,763 issued to Jack et al., as well as the exemplary manufacturing assemblies disclosed in co-pending, commonly owned U.S. patent application Ser. No. 10/016,524 entitled “Flexible Track Drilling Machine” filed Dec. 10, 2001, co-pending, commonly-owned U.S. patent application Ser. No. 10/606,402 entitled “Apparatus and Methods for Servo-Controlled Manufacturing Operations” filed Jun. 25, 2003, co-pending, commonly-owned U.S. patent application Ser. No. 10/606,443 entitled “Methods and Apparatus for Counter-Balance Assisted Manufacturing Operations” filed Jun. 25, 2003, co-pending, commonly-owned U.S. patent application Ser. No. 10/606,472 entitled “Methods and Apparatus for Manufacturing Operations Using Opposing-Force Support Systems” filed Jun. 25, 2003, and co-pending, commonly-owned U.S. patent application Ser. No. 10/606,473 entitled “Apparatus and Methods for Manufacturing Operations Using Non-Contact Position Sensing” filed Jun. 25, 2003, which patents and patent applications are hereby incorporated by reference.  
      For example,  FIG. 4  is an isometric view of a representative manufacturing assembly  400  in accordance with yet another embodiment of the invention. In this embodiment, the manufacturing assembly  400  includes a track assembly  410  controllably attachable to a workpiece  130 , and a carriage assembly  420  moveably coupled to the track assembly  410 . A sensing component  430  is mounted on the carriage assembly  420  and is operatively coupled to a controller  434 . At least one of the sensing component  430  and the controller  434  may also be coupled to a manufacturing tool  451  mounted on the carriage assembly  420 . As described more fully below, the sensing component  430  may include at least a portion of a sensing system in accordance with an embodiment of the present invention.  
      As further shown in  FIG. 4 , the track assembly  410  may include first and second rails  422 ,  424 , each rail  422 ,  424  being equipped with a plurality of vacuum cup assemblies  414 . The vacuum cup assemblies  414  are fluidly coupled to one or more vacuum lines  416  leading to a vacuum source  418 , such as a vacuum pump or the like, such that vacuum may be controllably applied to the vacuum cup assemblies  414  to secure the track assembly  410  to the workpiece  130 . The vacuum cup assemblies  414  are of known construction and may be of the type disclosed, for example, in U.S. Pat. No. 6,467,385 B1 issued to Buttrick et al., or U.S. Pat. No. 6,210,084 B1 issued to Banks et al. In alternate embodiments, the vacuum cup assemblies  414  may be replaced with other types of attachment assemblies, including magnetic attachment assemblies, bolts or other threaded attachment members, or any other suitable attachment assemblies.  
      The rails  422 ,  424  may be connected by one or more connecting members  428 , and may be adapted to bend, twist, and flex to adjust to the contours of the workpiece  130 . The carriage assembly  420  may translate along the rails  422 ,  424  by virtue of rollers  432  that are mounted on an x-axis carriage  460  of the carriage assembly  420  and engaged with the rails  422 ,  424 . In a particular embodiment, each rail  422 ,  424  may have a V-shaped edge engaged by the rollers  32 , and the rollers  32  may include V-shaped grooves that receive the V-shaped edges of the rails  422 ,  424 . In another embodiment, the x-axis carriage  460  may be adapted to flex and twist as needed (i.e. as dictated by the contour of the workpiece  130 ) as the carriage assembly  420  traverses the rails  422 ,  422  to allow a limited degree of relative movement to occur between the x-axis carriage  430  and the rollers  432 . Consequently, a reference axis of the carriage assembly  420  (in the illustrated embodiment, a z-axis normal to the plane of the x-axis carriage  460 ) may be maintained substantially normal to the workpiece  130  at any position of the carriage assembly  420  along the rails  422 ,  424 .  
      As further shown in  FIG. 4 , a rack  438  for a rack and pinion arrangement is mounted along the rail  424 . A first motor  440  and associated first gearbox  442  is mounted on the carriage assembly  420 . An output shaft from the first gearbox  442  has a first pinion gear  444  mounted thereon which engages the rack  438  on the rail  424 . Thus, rotation of the first pinion gear  444  by the first motor  440  drives the carriage assembly  420  along the rails  422 ,  424 .  
      With continued reference to  FIG. 4 , the carriage assembly  420  further includes a y-axis carriage  450  slideably mounted atop the x-axis carriage  460  so that the y-axis carriage  450  can slide back and forth along a y-axis direction perpendicular to the x-axis direction. More particularly, rails  452 ,  454  are affixed to the opposite edges of the x-axis carriage  460 , and rollers  456  are mounted on the y-axis carriage  450  for engaging the rails  452 ,  454 . A rack  458  for a rack and pinion arrangement is affixed to the x-axis carriage  460  along the rail  454 . A second motor  480  and associated second gearbox  482  are mounted on the y-axis carriage  450  and drive a second pinion gear (not shown) that engages the rack  458  to drive the y-axis carriage  450  in the y-axis direction.  
      In operation, the manufacturing assembly  400  may be mounted onto the workpiece  130  and the carriage assembly  420  may be moved to a desired position over the workpiece  130 . Specifically, the controller  434  may transmit control signals to the first drive motor  440  to drive the carriage assembly  420  along the track assembly  410 , and may also transmit control signals to the second drive motor  480  to adjust the position of the y-axis carriage  4  be coupled to the carriage assembly  420  by, for example, a clamp ring  470  or other suitable structure that provides access to the workpiece  130  for the manufacturing tool  451 .  
      It will be appreciated that the sensing component  430  may include a portion of a sensing system in accordance with an embodiment of the present invention. For example, in alternate aspects, the manufacturing assembly  400  may be mounted on the first surface  132  of the workpiece  130  (with or without the manufacturing tool  451 ), and the sensing component  430  may include the magnet  110  and field-directing polepiece  112  of the sensing system  100  ( FIG. 1 ), or the magnet  210  and field-directing polepiece  212  of the sensing system  200  ( FIG. 2 ), or the electromagnet  360  and the field-directing polepiece  312  of the sensing system  300  ( FIG. 3 ), or alternate embodiments thereof. Thus, the manufacturing assembly  400  may be used to index the first location  136  on the first surface  132  of the workpiece  130  and to position a shaped magnetic field portion which may be sensed by a magnetic field sensor from an opposing side of the workpiece  130 , as generally described above.  
      In alternate embodiments, however, the manufacturing assembly  400  may be mounted on the second surface  134 , and the sensing component  430  may include a magnetic field sensor (e.g. the sensor  120 ). Thus, in such alternate embodiments, the manufacturing assembly  400  may be employed to move the magnetic field sensor  120  along a traversing path to detect the shaped magnetic field portion extending through the workpiece  130 . The signals from the magnetic field sensor  120  may be transmitted to the controller  434 , which may determine the second location  140  on the second surface  134 , and may further transmit appropriate control signals to the first and second motors  440 ,  480 , and to the manufacturing tool  451  to perform a desired manufacturing operation at the second location  140 .  
      It should also be understood that the various operations of the manufacturing assembly  400  may be controlled by the controller  430 , and may be accomplished in an automated or semi-automated manner using computerized numerically-controlled (CNC) methods and algorithms. Alternately, the various operations of the manufacturing assembly  400  may be performed manually or partially-manually by an operator, such as, for example, by having the operator provide manual control inputs to the controller  434 , or by temporarily disabling or neutralizing the above-referenced motors and drive assemblies to permit manual movement. In a particular aspect, the controller  434  includes a CNC control system. It may also be noted that manufacturing assemblies in accordance with the present invention, including the manufacturing assembly  400  described above, may be operated in combination with a wide variety of manufacturing tools  451 , including but not limited to, drilling devices, riveters, mechanical and electromagnetic dent pullers, welders, wrenches, clamps, sanders, nailers, screw guns, or virtually any other desired type of manufacturing tools or measuring instruments.  
       FIG. 5  is a flowchart of a method  500  of performing a manufacturing operation including through-skin magnetic sensing in accordance with a further embodiment of the invention. In this embodiment, the method  500  begins at a block  502 , and a magnet is positioned at a desired indexing location relative to a first side of a workpiece at a block  504 . As described above, the magnet may be manually positioned, or alternately, may be positioned using an automated or semi-automated manufacturing assembly, or by any other suitable means. At a block  506 , a shaped magnetic field portion is generated which at least partially extends outwardly from a second side of the workpiece. The shaped magnetic field portion may be generated using one or more permanent magnets or electromagnets, in combination with one or more field-directing polepieces located on at least one of the first and second sides of the workpiece.  
      As further shown in  FIG. 5 , a magnetic field sensor may then be translated through at least a portion of the shaped magnetic field portion, and signals indicating a magnetic field strength may be sensed, at a block  508 . Again, as noted above, the magnetic field sensor may be translated using an automated or semi-automated manufacturing assembly, or manually. Furthermore, the traversing path of the magnetic field sensor may be constrained, such as by maintaining a constant distance to a surface of the workpiece, or alternately, may be traversed without regard to the normality or angular orientation of the traversing path with respect to the workpiece. Then, at a block  510 , a desired location for performing a manufacturing operation may be determined. This determination may include transmitting the sensed signals and analyzing the sensed signals using a controller or other suitable data analyzer.  
      As a block  512 , the manufacturing operation may be performed at the desired location. The manufacturing operation may, for example, be drilling, welding, riveting, or any other desired operation. Then, at a block  514 , a determination regarding whether the manufacturing operations are complete is made. If so, the method  500  proceeds to termination at a block  516 . Alternately, the method  500  may return to the block  504 , and the actions in blocks  504  through  514  may be iteratively repeated as needed until all desired manufacturing operations are accomplished.  
      While specific embodiments of the invention have been illustrated and described herein, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention should not be limited by the disclosure of the specific embodiments set forth above. Instead, the invention should be determined entirely by reference to the claims that follow.