Patent Publication Number: US-7723974-B1

Title: Planishing apparatus and method

Description:
This is a division of U.S. patent application Ser. No. 09/134,634, filed Aug. 14, 1998 now U.S. Pat. No. 6,123,249 by R. A. Venable, K. G. Ezell, and D. E. Hartley. 
   The invention described herein was made in the performance of work under NASA Contract No. 36200 and is subject to the provisions of section 305 of the National Aeronautics and Space Act of 1958 (42 U.S.C. 2457). 
   This application claims priority of Provisional Patent Application Ser. No. 60/083,398, filed Apr. 29, 1998. 

   FIELD OF THE INVENTION 
   This invention relates to techniques for planishing or stress relieving joints, welds or the like in substantially planar structures. 
   BACKGROUND OF THE INVENTION 
   When fabricating large structures such as external propellant tanks for the Space Shuttle, large sheets of aluminum alloy are curved and welded along joints to define the overall shape of the tanks. The welding is preferably performed by automatic welding machinery, such as, for example, that described in U.S. Pat. No. 5,483,039, issued Jan. 9, 1996 in the name of Gallagher. It has been found that stress relieving or planishing of the welded joint is advantageous, as described, for example, in U.S. patent application Ser. No. 08/803,481, filed Feb. 20, 1997 in the name of Shah et al. As described therein, planishing is accomplished by use of a planishing hammer applied to the weld on one side of the structure, and a backing or bucking bar held in a corresponding location on the other side of the structure. 
   It will be appreciated that the metal from which the propellant tank of the Space Shuttle is fabricated may, in some locations, be relatively thin, for weight minimization. During the planishing operation, the backing bar must be applied to the reverse side of the plates being welded, to prevent damage or actual puncturing of the plates by the hammer. As mentioned, the tanks are very large, so a human operator and either a backing bar or a power hammer can easily be accommodated within the tank, and the curvature of the plates is so large that, at any particular location, the joint or weld to be planished lies in an essentially planar structure. 
   It has been found to be difficult to reliably maintain the backing bar at a position on one side of the structure which corresponds to the location of the head of the planishing hammer. While the weld is visible from both sides of the structure, the exact position along the weld which is being planished must be identified to within the diameter of the backing bar. Attempts have been made to use radio communications to give instructions and information across the welded walls, but this has not proven to be effective. 
   Improved arrangements for identifying the location of a planishing hammer are desired. 
   SUMMARY OF THE INVENTION 
   Thus, an assemblage of parts according to an aspect of the invention is suited for planishing a preferably nonmagnetic, generally planar structure including first and second broad surfaces. The assemblage of parts comprises a planishing hammer including a body, a hammer head, and driving means coupled to the body and to the head, for driving the hammer head in a fore-and-aft direction in a reciprocating manner over a range of travel. In use, the head of the hammer is applied to or held against the first broad surface of the planar structure. The assemblage of parts includes a first magnet, and a first magnet support arrangement coupled to the body and to the first magnet, for supporting the first magnet at a fore-aft location, measured from the first surface adjacent the range of travel, which position is laterally displaced from the head relative to the fore-aft direction. As a result of support of the first magnet in this manner, the magnetic field of the first magnet penetrates the planar structure when the head is adjacent the first broad surface. The assemblage of parts further includes a second magnet adapted to be located on the second broad surface of the planar structure, and to be held in place against the second broad surface of the planar structure by the magnetic field of the first magnet. A backing piece or bar of the assemblage of parts is adapted to be held against the second broad surface at the joint or weld of the planar structure, at a location identified by the location of the second magnet. 
   In one embodiment of the invention, the first magnet support arrangement comprises an elongated rod extending in the fore-and-aft direction; the support arrangement includes fore and aft ends. The first rod terminates at the fore end at the first magnet. The first magnet support arrangement also includes an affixing arrangement coupled to the body and the rod. The affixing arrangement is affixed to the rod at a location lying between the fore and aft ends. 
   A method according to an aspect of the invention is for planishing an elongated, visible joint in a nonmagnetic, generally planar structure including first and second broad surfaces. The method includes the step of procuring an assemblage of parts similar to that described above. More particularly, the procuring step procures 
   (a) a planishing hammer including a body including a hammer head and driving arrangement coupled to the body and the head, for driving the hammer head in a fore-and-aft direction in a reciprocating manner over a range of travel; 
   (b) a first magnet; 
   (c) first magnet support arrangement coupled to the body and to the first magnet, for supporting the first magnet at a fore-aft location generally adjacent the range of travel, and laterally displaced from the head relative to the fore-aft direction; 
   (d) a second magnet; and 
   (e) a backing piece. 
   The method further comprising the step of, on a first side of the planar structure adjacent the first broad surface, holding the hammer with the head against the joint on the first broad surface of the planar structure, and with the first magnet at a location which is on a line orthogonal to the direction of elongation of the joint at the location of the head, whereby the magnetic field of the first magnet penetrates the planar structure to the second side thereof. The method also includes the further step of, on a second side of the planar structure adjacent the second broad surface, placing the second magnet on the second surface within the magnetic field of the first magnet, as a result of which the second magnet is attracted toward the first magnet. The backing piece is placed on the second broad surface at a location on the elongated joint at which a line extending from the second magnet to the elongated joint joins the joint orthogonally. An important step in this method is selection of the strength of the first and second magnets in conjunction with at least one of the thickness and the material of the structure as measured between the first and second broad surfaces, in such a manner that the second magnet is held against the second broad surface of the planar structure by the magnetic field produced by the first magnet. The planishing method further includes operation of the hammer so that the head strikes the joint on the first surface of the structure. 
   According to another manifestation of the invention, a sensor arrangement providing an indication in one dimension of the location of a hidden magnet. The sensor comprises a set of a plurality of magnetic sensors arrayed in a straight line in an array direction. Each of the magnetic sensors is capable of responding to the strength of a magnetic field by adopting or changing an electrical characteristic. A set of a plurality, no less in number than the number of the plurality of magnetic sensors, of electrically actuated indicators is arrayed in a direction parallel to the array direction. A source of electrical energy, which may be a battery, is provided, and a control arrangement is coupled to the source, to the arrayed set magnetic sensors and to the arrayed set of indicators, for providing an indication of the position along the array of magnetic sensors at which the magnetic field is greatest. In a preferred embodiment of this manifestation, each of the electrically actuated indicators comprises a solid-state light emitter, which may be a light-emitting diode or a laser. The preferred magnetic sensors include solid-state devices such as Hall-effect devices or Giant Magneto-Resistive sensors. In a particular version of this manifestation, the number of the plurality of the magnetic sensors exceeds two, and the control arrangement comprises an array of electrical conductors. The array of electrical conductors includes individual ones of the electrical conductors which are associated only with an individual one of the magnetic sensors and with a corresponding associated one of the indicators, for allowing the flow of current through the one of the magnetic sensors and the associated one of the indicators, but not through others of the magnetic sensors and indicators. In another version of this manifestation, the number of the plurality of the magnetic sensors in the sensor arrangement is two, and the control arrangement comprises a differential processing arrangement coupled to the source of electrical energy, to the magnetic sensors, and to the indicator arrangement. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIGS. 1   a ,  1   b , and  1   c  are simplified God&#39;s-eye, plan, and front elevation views, respectively, of a nonmagnetic planar workpiece, planishing hammer, and sensor according to an aspect of the invention; 
       FIGS. 2   a ,  2   b , and  2   c  are simplified God&#39;s-eye, plan, and front elevation views, respectively, of a planar workpiece, planishing hammer, and sensor according to another manifestation of the invention; 
       FIGS. 2   d  and  2   e  are simplified front and rear elevation views of a structure including a joint, illustrating another method for use of the sensor of  FIGS. 2   a ,  2   b , and  2   c;    
       FIG. 2   f  is a simplified schematic diagram of the sensor of  FIGS. 2   a ,  2   b , and  2   c;    
       FIG. 3   a  is a simplified diagram of a planishing hammer with a first magnet located in the hammer head,  FIG. 3   b  is a simplified depiction of a backing bar with a two-magnetic-sensor arrangement affixed thereto for sensing the relationship of the backing bar to the magnet, and  FIG. 3   c  is a simplified schematic diagram illustrating one possible way to produce an indication from the two sensors on the backing bar. 
   

   DESCRIPTION OF THE INVENTION 
   In  FIGS. 1   a  and  1   b , an assemblage  10  of parts includes a planishing hammer  12  with a body  14  and a hammer head  16 . Hammer head  16  is driven in a reciprocating manner in a direction illustrated by arrows  19  by a motor  17  which is mechanically coupled to the body  14  and to the hammer head  16 . Motor  17  is powered by a source (not illustrated). A magnet  20  is supported by an elongated support rod  22  defining a fore end  22   f  and an aft end  22   a . Support rod  22  extends parallel to the direction  19  of reciprocating motion of the hammer head  16 , and is fastened to the hammer body  14  by an attachment arrangement  18 , which includes first and second attachments  18   a  and  18   b . Attachments  18   a  and  18   b  couple to the support rod  22  at a location lying between fore end  22   f  and aft end  22   a.    
   Assemblage of elements  10  further includes a second magnet  24  and a backing or bucking bar  26 . As illustrated in  FIG. 1   a , a nonmagnetic plate or generally planar structure  30  is seen in God&#39;s eye view, which is to say a view in which both broad sides  31  and  32  are simultaneously visible. As illustrated, planar structure  30  has a vertically oriented elongated weld  34 , which defines an axis  8  of elongation. The hammer  12  is located on a first side  1  which faces first surface  31  of planar structure  30 , with hammer head  16  applied to a location along the weld  34 . The second magnet  24  and backing bar  26  are located on a second side  2  of the planar structure  30 . The second side  2  faces the second surface  32  of the planar structure  30 . The backing bar  26  is applied to weld  34  on second surface  32  at a location corresponding to that at which hammer head  16  is applied to the first surface  31 . 
   As mentioned above, it is difficult to determine the exact location to which the backing bar should be applied. It must be appreciated that the planar structure  30  is part of a large tank, which prevents communication around sides of the structure, which are illustrated for ease in representation. According to a first aspect of the invention, the support rod  22  is positioned so that magnet  20  is located near the forward end of the range of travel T of hammer head  16 , as illustrated in  FIG. 1   b . The exact axial position at which first magnet  20  should be positioned will depend, in part, on the hammer reciprocation speed. If the reciprocation speed is high, the hammer body will be held at a distance from the first surface  31  which corresponds to the maximum excursion of the hammer head, represented in  FIG. 1   b  by phantom head position  16   2 , in which case magnet support rod  22  can be positioned so that first magnet  20  lies just behind, or away from the surface  31 . On the other hand, if the hammer repetition rate were very low, the hammer body would make slow excursions toward and away from the surface  31  of structure  30 , in which case the rod  22  would have to be positioned so that the first magnet  20  was slightly behind the aft-most position of the hammer, in order to avoid hitting the first surface  31 . Hitting of the surface  31  by the first magnet  20  is not per se objectionable, but might chip the magnet, damage the surface, or make it difficult to control the hammer&#39;s position. 
   First magnet  20  is selected to be sufficiently strong to produce a magnetic field  40  which penetrates through the planar structure in the context of a nonmagnetic structure, or to produce a suitable magnetic field on the second side of the structure, and to hold the second magnet in place adjacent the first magnet. The preferred type of magnet  20  is made from Neodymium Iron Boron. 
   In a method according to the invention, the backing bar is located on the second side  2 , and is applied to the second surface  32  of the planar structure  30  at a location selected as described in conjunction with  FIG. 1   c . As illustrated in the top or plan view of  FIG. 1   b , the hammer  12  is held so that the first magnet  20  is to one side of the joint  34 . More particularly, the hammer is held so that first magnet  20  is at a location corresponding to location  241  of  FIG. 1   c . Location  241  is the location at which second magnet  24  is held by the magnetic field of first magnet  20 . As illustrated in  FIG. 1   c , location  241  lies on an imaginary line  50  which is orthogonal to the axis  8  of joint  34 , as suggested by right-angle symbol  52 . With this positioning of the hammer  12  and first magnet  20 , the second magnet  24  gives a visual indication of the location  54  to which the backing bar  26  should be applied so as to directly behind the location to which the hammer head  16  is applied. The vibration of the structure occasioned by operation of hammer  12  provides second magnet  24  with the slight mechanical energy which is required to allow it to move to follow the changing position of hammer  12  and first magnet  20  during planishing. Thus, the second magnet  24  moves with the hammer  12 , and the location to which the backing bar  26  is applied is simply the orthogonal projection from the location of the second magnet  24  to the joint  34 . 
     FIGS. 2   a ,  2   b , and  2   c  are similar to  FIGS. 1   a ,  1   b , and  1   c , and corresponding elements of the FIGURES are designated by the same reference numerals. In  FIGS. 2   a ,  2   b , and  2   c , the magnetic sensor, instead of being a simple magnet  24 , is a sensor arrangement designated  224 . As illustrated, sensor arrangement  224  is elongated, and bears an array  200  of magnetic field indicators  201 ,  202 ,  203 ,  204 ,  205 ,  206 ,  207 ,  208 ,  209 , and  210 . One of the indicators, namely indicator  203 , is illustrated as being illuminated (or conversely, darkened) in  FIG. 2   c . This identifies the location of the strongest portion of the magnetic field generated by first magnet  20  applied to the first side  31  of structure  30 . The location  254  to which the backing bar  26  should be applied is determined from the illuminated (or darkened) one of the indicators of set  200  of magnetic field or strength indicators, by orthogonally (symbol  252 ) projecting a line  250  from the illuminated indicator  203  to the axis  8  of elongation of the joint  34 . 
     FIGS. 2   d  and  2   e  illustrate the relationships on sides  1  and  2 , respectively, of planar structure  30 , for a different orientation of the magnetic sensor arrangement  224 . As illustrated in  FIG. 2   e , the sensor arrangement  224  is attached to that end of the backing bar which is adjacent to the second surface  32  of the planar structure, and is held with the sensor array oriented parallel with the axis  8  of elongation of the joint  34 , which is in the vertical direction in  FIG. 2   e . As illustrated in  FIG. 2   d , the hammer  12  is held so the magnet  20  lies directly over the joint  34  being planished. The magnetic field of the magnet  20  extends through the joint, affects the array of magnetic sensors associated with sensor arrangement  224 . As in the case of the embodiment of  FIGS. 2   a ,  2   b , and  2   c , the sensor arrangement  224  responds by illuminating or darkening an appropriate one of the indicators, to thereby give an indication of where the magnet is located on the first side  1 . In this case, the backing bar  26  is moved up or down along the joint until a particular one of the indicators of array  200  is illuminated or darkened; it should preferably be one in the center of the array, as for example indicator  205 . 
     FIG. 2   f  is a simplified schematic diagram of one embodiment of a sensor arrangement  224 . In  FIG. 2   f , a set  262  of individual Hall-effect magnetic field sensors  262   a ,  262   b ,  262   c , . . .  262   d ,  262   e ,  262   f ,  262   g ,  252   h  is line-arrayed parallel to a line  280 . The Hall sensors are energized from a source of electrical energy illustrated as a battery  266 , by way of two conductors or buses  268 ,  270 . A set  264  of indicators  264   a ,  264   b ,  264   c , . . .  264   d ,  264   e ,  264   f ,  264   g ,  264   h  is similarly arrayed parallel to line  280 . An interconnection arrangement including a common conductor  272  and a set  274  of a plurality of individual conductors  274   a ,  274   b ,  274   c , . . .  274   d ,  274   e ,  274   f ,  274   g , and  274   h  interconnects the individual members of the arrayed set  262  of magnetic sensors with corresponding individual members of the set  264  of indicators. More particularly, conductor  274   a  interconnects sensor  262   a  with indicator  264   a , conductor  274   b  interconnects sensor  262   b  with indicator  264   b , conductor  274   c  interconnects sensor  262   c  with indicator  264   c , conductor  274   d  interconnects sensor  262   d  with indicator  264   d , conductor  274   e  interconnects sensor  262   e  with indicator  264   e , conductor  274   f  interconnects sensor  262   f  with indicator  264   f , conductor  274   g  interconnects sensor  262   g  with indicator  264   g , and conductor  274   h  interconnects sensor  262   h  with indicator  264   h . These individual connections control or assure that sensing of a magnetic field by one of the sensors  262   a ,  262   b ,  262   c , . . .  262   d ,  262   e ,  262   f ,  262   g ,  252   h  illuminates only the corresponding one of the indicators  264   a ,  264   b ,  264   c , . . .  264   d ,  264   e ,  264   f ,  264   g ,  264   h.    
     FIG. 3   a  illustrates a planishing hammer  12  including a body  14  and a hammer head  16 . A magnet  320  is centered in hammer head  16 , so its magnetic field can be sensed. In  FIG. 3   b , a backing bar  326  bears a sensor box  324  which is to be held above the backing bar. The backing bar also bears a set  300  of two Giant Magneto-Resistive (GMR) magnetic sensors  301  and  302 , located above and below the fore end  326   f  of the bar. When the backing bar  326  is correctly positioned relative to the hammer head  16  of  FIG. 3   a , GMR sensors  301  and  302  are located above and below magnet  320  by the same distance, and so have the same resistance. When one sensor  301 ,  302  is closer to the magnet  320  than the other, their resistances will differ. 
     FIG. 3   c  is a simplified schematic diagram of a differential sensing arrangement for providing an indication of the relative positioning of the backing bar and the hammer head/magnet. In  FIG. 3   c , a battery  366  is supplies power to a Wheatstone bridge circuit  326 , which includes first and second equal-value fixed resistors  328 ,  330  connected to the positive terminal of the battery and to the sensing terminals  340 ,  342 , respectively, of the bridge. The GMR sensors  301 ,  302  are connected from the negative battery terminal (by way of ground) to sensing terminals  340 ,  342 , respectively. A differential indicator in the form of a galvanometer  332  is coupled across the sensing terminals. Deflection of the needle of the galvanometer in one direction or the other indicates that the GMR sensors are unbalanced, and that the backing bar should be moved in the appropriate direction to equalize the magnetic fields at the GMR sensors, to thereby center the backing bar on the magnet  320 . For ease of interpretation, the galvanometer  332  is preferably mounted so that the deflection of the needle indicates the direction in which the backing bar should be moved. 
   Other embodiments of the invention will be apparent to those skilled in the art. For example, while Hall-effect and other solid-state sensors are described, the magnetic field sensors might be as simple as magnetically actuated reed switches. The substantially planar structure  30  may be made from aluminum alloy, such as aluminum-lithium alloy, or from any other material which allows the magnetic field  40  of the first magnet  20  to penetrate to the second side  2  with sufficient strength to hold the second magnet  24  in place; this might even be a thin sheet of “soft” steel, which would allow sufficient magnetization of the material to occur so as to produce a suitable magnetic field on the second side  32  of the structure. While the number of magnetic field indicators associated with sensor arrangement  224  has been described as ten, this number is arbitrary, and could be greater or less. While a simple galvanometer circuit has been illustrated for providing a differential indication from the two-sensor version of  FIGS. 3   a  and  3   b , those skilled in the art know that many circuit configurations may be used, including electronic processing devices, and similarly that a multiple-indicator array may be used with a two-sensor indicator, if desired. 
   Thus, an assemblage of parts ( 10 ) according to an aspect of the invention is suited for planishing a preferably nonmagnetic, generally planar structure ( 30 ) including first ( 31 ) and second ( 32 ) broad surfaces. The assemblage of parts ( 10 ) comprises a planishing hammer ( 12 ) including a body ( 14 ), a hammer head ( 16 ), and driving means ( 17 ) coupled to the body ( 14 ) and to the head ( 16 ), for driving the hammer head ( 16 ) in a fore-and-aft direction ( 19 ) in a reciprocating manner over a range of travel (T). In use, the head ( 16 ) of the hammer ( 12 ) is applied to or held against the first broad surface ( 31 ) of the planar structure ( 30 ). The assemblage of parts includes a first magnet ( 20 ), and a first magnet support arrangement ( 18 ,  22 ) coupled to the body ( 14 ) and to the first magnet ( 20 ), for supporting the first magnet ( 20 ) at a fore-aft location (d, measured from the first surface  31 ) adjacent the range (T) of travel, which position is laterally displaced from the head relative to the fore-aft direction ( 19 ). As a result of support of the first magnet ( 20 ) in this manner, the magnetic field ( 40 ) of the first magnet ( 20 ) penetrates the planar structure ( 30 ) when the head ( 16 ) is adjacent the first broad surface ( 31 ). The assemblage of parts ( 10 ) further includes a second magnet ( 24 ) adapted to be located on the second broad surface ( 32 ) of the planar structure ( 30 ), and to be held in place against the second broad surface ( 32 ) of the planar structure ( 30 ) by the magnetic field ( 40 ) of the first magnet ( 20 ). A backing piece or bar ( 26 ) of the assemblage of parts ( 10 ) is adapted to be held against the second broad surface ( 32 ) at the joint or weld ( 34 ) of the planar structure ( 30 ), at a location identified by the location of the second magnet ( 24 ). 
   In one embodiment of the invention, the first magnet ( 20 ) support arrangement ( 18 ,  22 ) comprises an elongated rod ( 22 ) extending in the fore-and-aft direction ( 19 ); the support arrangement includes fore ( 20   f ) and aft ( 20   a ) ends. The first rod ( 20 ) terminates at the fore end ( 20   f ) at the first magnet ( 20 ). The first magnet support arrangement also includes 
   an affixing arrangement ( 18 ) coupled to the body ( 14 ) and the rod ( 20 ). The affixing arrangement ( 18 ) is affixed to the rod ( 20 ) at a location lying between the fore ( 20   f ) and aft ( 20   a ) ends. 
   A method according to an aspect of the invention is for planishing an elongated, visible joint ( 34 ) in a nonmagnetic, generally planar structure ( 30 ) including first ( 31 ) and second ( 32 ) broad surfaces. The method includes the step of procuring an assemblage of parts similar to that described above. More particularly, the procuring step procures (a) a planishing hammer ( 12 ) including a body ( 14 ) including a hammer head ( 16 ) and driving arrangement ( 17 ) coupled to the body ( 14 ) and the head ( 16 ), for driving the hammer head ( 16 ) in a fore-and-aft direction ( 19 ) in a reciprocating manner over a range of travel (T); 
   (b) a first magnet ( 20 ); 
   (c) first magnet ( 20 ) support arrangement ( 18 ,  22 ) coupled to the body ( 14 ) and to the first magnet ( 20 ), for supporting the first magnet ( 20 ) at a fore-aft location (d) generally adjacent the range of travel, and laterally displaced from the head relative to the fore-aft direction ( 19 ); 
   (d) a second magnet ( 24 ); and 
   (e) a backing piece ( 26 ). 
   The method further comprising the step of, on a first side ( 1 ) of the planar structure ( 30 ) adjacent the first broad surface ( 31 ), holding the hammer ( 12 ) with the head ( 16 ) against the joint ( 34 ) on the first broad surface ( 31 ) of the planar structure ( 30 ), and with the first magnet ( 20 ) at a location ( 241 ) which is on a line ( 50 ) orthogonal ( 52 ) to the direction of elongation ( 8 ) of the joint ( 34 ) at the location of the head ( 16 ), whereby the magnetic field ( 40 ) of the first magnet ( 20 ) penetrates the planar structure ( 30 ) to the second side ( 2 ) thereof. The method also includes the further step of, on a second side ( 2 ) of the planar structure ( 30 ) adjacent the second broad surface ( 32 ), placing the second magnet ( 24 ) on the second surface ( 32 ) within the magnetic field ( 40 ) of the first magnet ( 20 ), as a result of which the second magnet ( 24 ) is attracted toward the first magnet ( 20 ). The backing piece ( 26 ) is placed on the second broad surface ( 32 ) at a location ( 54 ) on the elongated joint ( 34 ) at which a line ( 50 ) extending from the second magnet ( 24 ) to the elongated joint ( 34 ) joins the joint ( 34 ) orthogonally ( 52 ). An important step in this method is selection of the strength of the first and second magnets ( 24 ) in conjunction with at least one of the thickness (t) and the material of the structure ( 30 ) as measured between the first ( 31 ) and second ( 32 ) broad surfaces, in such a manner that the second magnet ( 24 ) is held against the second broad surface ( 32 ) of the planar structure ( 30 ) by the magnetic field ( 40 ) produced by the first magnet ( 20 ). The planishing method further includes operation of the hammer ( 12 ) so that the head ( 16 ) strikes the joint ( 34 ) on the first surface ( 31 ) of the structure ( 30 ). 
   According to another manifestation of the invention, a sensor arrangement ( 224 ) providing an indication in one dimension of the location of a hidden magnet. The sensor ( 224 ) comprises a set ( 262 ) of a plurality (eight illustrated in  FIG. 2   f ) of magnetic sensors ( 262   a ,  262   b ,  262   c ,  262   d ,  262   e ,  262   f ,  262   h ) arrayed in a straight line ( 280 ) in an array direction ( 280 ). Each of the magnetic sensors ( 262   a ,  262   b ,  262   c ,  262   d ,  262   e ,  262   f ,  262   h ) is capable of responding to the strength of a magnetic field ( 40 ) by adopting or changing an electrical characteristic. A set ( 264 ) of a plurality (eight illustrated in  FIG. 2   e ), no less in number than the number (eight) of the plurality of magnetic sensors ( 262   a ,  262   b ,  262   c ,  262   d ,  262   e ,  262   f ,  262   h ), of electrically actuated indicators ( 264   a ,  264   b ,  264   c ,  264   d ,  264   e ,  264   f ,  264   h ) is arrayed in a direction parallel to the array direction ( 280 ). A source ( 266 ) of electrical energy, which may be a battery, is provided, and a control arrangement ( 268 ,  270 ,  272 ,  274 ) is coupled to the source, to the arrayed set ( 262 ) magnetic sensors ( 262   a ,  262   b ,  262   c ,  262   d ,  262   e ,  262   f ,  262   h ) and to the arrayed set ( 264 ) of indicators ( 264   a ,  264   b ,  264   c ,  264   d ,  264   e ,  264   f ,  264   h ), for providing an indication of the position along the array ( 262 ) of magnetic sensors ( 262   a ,  262   b ,  262   c ,  262   d ,  262   e ,  262   f ,  262   h ) at which the magnetic field ( 40 ) is greatest. In a preferred embodiment of this manifestation, each of the electrically actuated indicators ( 264   a ,  264   b ,  264   c ,  264   d ,  264   e ,  264   f ,  264   h ) comprises a solid-state light emitter, which may be a light-emitting diode or a laser. The preferred magnetic sensors ( 262   a ,  262   b ,  262   c ,  262   d ,  262   e ,  262   f ,  262   h ) include solid-state devices such as Hall-effect devices or Giant Magneto-Resistive sensors ( 262 ). In a particular version ( FIG. 2   e ) of this manifestation, the number (eight illustrated in  FIG. 2   e ) of the plurality of the magnetic sensors ( 262   a ,  262   b ,  262   c ,  262   d ,  262   e ,  262   f ,  262   h ) exceeds two, and the control arrangement ( 268 ,  270 ,  272 ,  274 ) comprises an array of electrical conductors. The array of electrical conductors includes individual ones ( 274   a ,  274   b ,  274   c ,  274   d ,  274   e ,  274   f ,  274   g ,  274   h ) of the electrical conductors which are associated only with an individual one ( 262   a ,  262   b ,  262   c ,  262   d ,  262   e ,  262   f ,  262   h , respectively) of the magnetic sensors and with a corresponding associated one of the indicators ( 264   a ,  264   b ,  264   c ,  264   d ,  264   e ,  264   f ,  264   h , respectively), for allowing the flow of current through the one of the magnetic sensors ( 262   a ,  262   b ,  262   c ,  262   d ,  262   e ,  262   f ,  262   h ) and the associated one of the indicators ( 264   a ,  264   b ,  264   c ,  264   d ,  264   e ,  264   f ,  264   h ), but not through others of the magnetic sensors ( 262   a ,  262   b ,  262   c ,  262   d ,  262   e ,  262   f ,  262   h ) and indicators ( 264   a ,  264   b ,  264   c ,  264   d ,  264   e ,  264   f ,  264   h ). In another version of this manifestation, the number of the plurality of the magnetic sensors ( 301 ,  302 ) in the sensor arrangement is two, and the control arrangement comprises a differential processing arrangement ( 326 ) coupled to the source ( 366 ) of electrical energy, to the magnetic sensors ( 301 ,  302 ), and to the indicator arrangement ( 332 ).