Abstract:
A multi-target photogrammetric target assembly and related method of evaluating curvilinear surface character. The target assembly includes a first photogrammetric target disposed at a first support and a second photogrammetric target disposed at a second support. The first support and the second support are operatively connected such that the first target is in predefined lateral spaced relation to the second target. The method includes providing a structure having a curvilinear surface and affixing one or more multi-target photogrammetric target assemblies to the curvilinear surface. The position of the targets is measured by one or more imaging devices to define surface contour characteristics.

Description:
TECHNICAL FIELD 
     This patent disclosure relates generally to photogrammetric targets for use in optical evaluation of surface contours and, more particularly, to photogrammetric target assemblies incorporating pairs of target structures disposed in spaced relation and adapted for placement on contoured surfaces to conduct measurement of those surfaces. 
     BACKGROUND 
     It is generally known to use light reflective targets in photogrammetric measurement of surfaces. According to this practice, the light reflective targets are positioned in a defined pattern on a surface to be measured and the positional relationship of the targets is measured and/or verified. Specifically, in a typical system, a measurement technician mounts multiple reflective targets on the surface to be measured and one or more cameras are then used to image the targets on a two-dimensional medium such as a film or a digital image sensor. A process of triangulation is then used to determine the relative three-dimensional positions of the targets. This permits development of a three dimensional image of the targeted surface. In the field of process control, such imaging may be used to compare the plotted positions of the targets on a formed part against a pre-established map of the specified formed part contours. This comparison is used to determine whether the formed part complies with given specification tolerances. One deficiency in such measurement systems is their dependence on manual placement of individual targets. In particular, in the event that the imaging technician fails to place targets properly the measurement will be in error. 
     Reflective targets for use in photogrammetric imaging are disclosed, for example, in U.S. Pat. No. 5,073,005 to Hubbs, having a filing date of May 2, 1988, and an issue date of Dec. 17, 1991. As best understood, this reference discloses target assemblies having a body with a mounted support surface adapted to hold a retro-reflective target. In one disclosed embodiment, the target body is elongated and provides a pair of target support surfaces at opposing ends. These target assemblies may be mounted on tooling or other surfaces for photogrammetric imaging evaluation. When using existing target assemblies, good results have been difficult to achieve due to difficulties in precisely aligning individual targets. Achieving good results may be particularly difficult when measuring surfaces such as boreholes and shaft surfaces. 
     SUMMARY 
     In accordance with one aspect, the present disclosure provides a photogrammetric target assembly. The target assembly includes a first photogrammetric target disposed at a terminal show surface of a first support pillar. The target assembly further includes a second photogrammetric target disposed at a terminal show surface of a second support pillar. The first support pillar and the second support pillar are operatively connected such that the first photogrammetric target is in predefined lateral spaced relation to the second photogrammetric target. 
     In accordance with another aspect, the present disclosure provides a method for measuring the contour of a curvilinear surface. The method includes providing a structure having a curvilinear surface and affixing a multi-target photogrammetric target assembly to the curvilinear surface. The photogrammetric target assembly includes a first photogrammetric target disposed at a terminal show surface of a first support pillar. The photogrammetric target assembly further includes a second photogrammetric target disposed at a terminal show surface of a second support pillar. The first support pillar and the second support pillar are operatively connected such that the first photogrammetric target is in predefined lateral spaced relation to the second photogrammetric target. The position of the first photogrammetric target and the second photogrammetric target are measured relative to one another. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view illustrating a cylindrical open bore structure; 
         FIG. 2  is a schematic view illustrating a photogrammetric target assembly incorporating a cooperative pair of target structures adapted for placement across a curved surface; 
         FIG. 3  is a schematic view illustrating placement of the target assembly of  FIG. 2  around the interior and exterior walls of the cylindrical open bore structure of  FIG. 1 ; 
         FIG. 4  is a schematic view taken generally along line  4 - 4  in  FIG. 2 , illustrating a potential connective relation between photogrammetric target support structures; 
         FIG. 5  is a schematic view similar to  FIG. 2 , illustrating another connective arrangement between photogrammetric target support structures; 
         FIG. 6  is a schematic view similar to  FIG. 4 , illustrating another connective arrangement between photogrammetric target support structures; 
         FIG. 7  is a schematic view similar to  FIG. 4 , illustrating yet another connective arrangement between photogrammetric target support structures; and 
         FIG. 8  is a schematic view illustrating a bi-lobal structure of unitary construction for a cooperative pair of photogrammetric targets. 
     
    
    
     DETAILED DESCRIPTION 
     As will be described further hereinafter, an improved photogrammetric target assembly is provided. The improved assembly includes a pair of reflective photogrammetric targets disposed at terminal show surfaces of operatively connected and laterally spaced supports. The improved assembly is adapted for measurement of curvilinear surfaces. 
     Reference will now be made to the drawings, wherein to the extent possible, like elements are designated by like reference numerals throughout the various views.  FIG. 1  illustrates an exemplary formed structural element  10  of curved surface geometry. The illustrated structural element  10  is in the form of an open bore cylinder including an axial bore  12  and a surrounding body  14  extending in ring-forming relation around the axial bore  12 . The structural element  10  illustrated in  FIG. 1  includes an inner curved surface  16  defining the boundary between axial bore  12  and body  14 . The illustrated structural element  10  also includes an outer curved surface  18  defining the exterior of body  14 . 
     It is to be understood and appreciated that the illustrated structural element  10  is exemplary only and is provided to generally illustrate structures of three-dimensional curved surface geometry as may be measured by target assemblies and measurement techniques consistent with the present disclosure as described more fully below. Accordingly, while open cylindrical structures such as the structural element  10  illustrated in  FIG. 1  may be well-suited for measurements using such target assemblies and techniques, other structures of curved surface geometry are likewise suitable for measurement by such target assemblies and techniques. 
       FIG. 2  illustrates an exemplary target assembly  20  of dual pillar construction adapted for use in photogrammetric measurement of curved surface structures. As shown, in the embodiment illustrated in  FIG. 2 , the target assembly includes a first reflective target  22 , and a second reflective target  24 . The first reflective target  22  is mounted in substantially centered relation at a first terminal show surface  25  disposed at one end of a first support pillar  26 . Likewise, the second reflective target  24  is mounted in lateral relation to the first reflective target  22  at a second terminal show surface  27  disposed at one end of a second support pillar  28 . In the illustrated target assembly  20 , a first attachment element  30  is located at an outer surface of first support pillar  26 . A second attachment element  32  may be located at the outer surface of second support pillar  28 . Additional attachment elements may likewise be utilized if desired. 
     According to one contemplated practice, the first attachment element  30  and the second attachment element  32  may be in the form of magnets to provide a releasable attachment connection with a structural element  10  of ferrous or other material characterized by magnetic attraction. However, it is also contemplated that other attachment systems such as adhesive systems and the like may be utilized if desired. It is also contemplated that combinations of different attachment systems may be utilized if desired. Moreover, while first support pillar  26  and second support pillar  28  are illustrated as having single attachment elements, it is likewise contemplated that multiple attachment elements may be utilized if desired. 
     According to the illustrated exemplary construction, first support pillar  26  and second support pillar  28  incorporate bodies of substantially circular cross-sectional geometry adjoined by a connector  34  such as a molded-in bridge or the like disposed at one or more positions along their lengths. In this regard, although first support pillar  26  and second support pillar  28  are each illustrated as having a generally circular cross-section, it is likewise contemplated that other cross-sectional geometries may also be utilized if desired. The support pillars  26 ,  28  and connector  34  may be formed of similar or dissimilar materials. By way of example only, and not limitation, each of the support pillars  26 ,  28  and connector  34  may be formed from moldable plastic. However, metals and other materials of construction may likewise be utilized if desired. 
     Regardless of the cross-sectional geometry of the support pillars, it is contemplated that bodies of the support pillars may be characterized by curved outer surfaces such that first support pillar  26  presents a first lobed surface  40  of convex curved profile and second support pillar  28  presents a second lobed surface  42  of convex curved profile. As best illustrated through simultaneous reference to  FIGS. 2 and 4 , the lobed surfaces  40 ,  42  may be disposed in spaced-apart, lateral relation on either side of connector  34  such that connector  34  provides a connection spanning the cusp between the first lobed surface  40  and the second lobed surface  42 . The spaced lobed surfaces  40 ,  42  cooperatively define a double lobed face  44  adapted for placement in contacting relation across a curved surface to be evaluated as described further hereinafter. 
     As noted previously, target assembly  20  may be used as part of a photogrammetric system to evaluate the configuration of curved surfaces in formed parts. In this regard, target assembly  20  is adapted for use in the evaluation of surface structures of convex or concave configuration. By way of example only, and not limitation,  FIG. 3  illustrates the placement of a plurality of target assemblies  20  around the inner wall and outer wall of a structural element  10  having a substantially tubular construction as previously described in relation to  FIG. 1 . As shown, the double lobed faces  44  of target assemblies  20  are disposed in contacting relation to the substantially concave inner curved surface  16 . The double lobed faces  44  of target assemblies  20  are also disposed in contacting relation to the substantially convex outer curved surface  18 . The position of the target assemblies  20  is maintained by releasable attachment elements  30 ,  32  ( FIG. 2 ) such as magnets or the like as previously described. Of course, it is to be understood that the actual arrangement of target assemblies  20  across the surfaces of structural element  10  is exemplary only. Thus, spacing and arrangement of target assemblies  20  can be varied as desired to evaluate a single surface or multiple surfaces within a structure. 
     Regardless of the nature of the structure being evaluated, target assemblies  20  may be arranged to present an array of reflective targets  22 ,  24  detectable by an imaging device  50  such as a camera adapted to record the relative positions of the targets  22 ,  24 . The use of three or more targets may be utilized to define a reference plane. The use of substantial numbers of targets may be utilized in some instances to define complex surfaces. According to one exemplary process, imaging device  50  is used to image the targets on a two-dimensional medium such as a film or a digital image sensor. A process of triangulation is then used to determine the relative three-dimensional positions of the targets. The plotted positions of the targets on the formed parts may then be compared against a pre-established map of the specified formed part contours. A target having a position which is inconsistent with pre-established specifications indicates the presence of an irregularity at that target position. 
     The double pillar structure of target assembly  20  may facilitate placement of target assembly  20  across a surface to be evaluated in a manner which promotes self aligning relative to the curve being measured, In particular, the operatively connected, laterally-spaced support pillars may provide a pair of reflective targets  22 ,  24  in established spaced-apart relation thereby reducing variability in the measurement process. Moreover, the double lobed face  44  also allows the target assembly  20  to establish and maintain contact with curved surfaces. 
     A number of different physical constructions may characterize target assemblies consistent with this disclosure. By way of example only, and not limitation,  FIG. 5  is a schematic view similar to  FIG. 2 , illustrating an alternative connective arrangement between photogrammetric target support structures. In  FIG. 5 , elements corresponding to those previously described are designated by like reference numerals within a 100 series. As shown, in the embodiment illustrated in  FIG. 5 , the target assembly  120  includes a first reflective target  122 , and a second reflective target  124 . The first reflective target  122  is mounted in substantially centered relation at a first terminal show surface  125  disposed at one end of a first support pillar  126 . Likewise, the second reflective target  124  is mounted in lateral relation to the first reflective target  122  at a second terminal show surface  127  disposed at one end of a second support pillar  128 . 
     According to the exemplary construction illustrated in  FIG. 5 , first support pillar  126  and second support pillar  128  incorporate bodies of substantially circular cross-sectional geometry adjoined by a connector  134  in the form of an extended width spacer. As in the configuration of  FIG. 2 , first support pillar  126  presents a first lobed surface  140  of convex curved profile and second support pillar  128  presents a second lobed surface  142  of convex curved profile. The lobed surfaces  140 ,  142  are disposed in substantially spaced-apart, lateral relation on either side of connector  134 . The support pillars  126 ,  128  and connector  134  may be formed of similar or dissimilar materials. By way of example only, and not limitation, each of the support pillars  126 ,  128  and connector  134  may be formed from moldable plastic. However, metals and other materials of construction may likewise be utilized if desired. 
     By way of example only, and not limitation,  FIG. 6  is a schematic view similar to  FIG. 4 , illustrating an alternative connective arrangement wherein elements corresponding to those previously described are designated by like reference numerals within a 200 series. In the construction of  FIG. 6 , the first support pillar  226  and the second support pillar  228  are operatively connected in spaced-apart relation using a substantially block shaped connector  234 . As illustrated, the connector  234  is adapted to engage adjustable coupling members  260  such as bolts or the like to provide an operative connection between the connector  234  and the support pillars. As will be appreciated, in such a structure, the insertion length of the coupling members  260  into the connector may be adjusted thereby adjusting the span between first support pillar  226  and the second support pillar  228 . Thus, the distance between the targets may be adjusted as desired. 
       FIG. 7  is a schematic view similar to  FIG. 4 , illustrating yet another alternative construction wherein elements corresponding to those previously described are designated by like reference numerals within a 300 series. In the construction of  FIG. 7 , the first support pillar  326  and the second support pillar  328  are operatively connected in spaced-apart relation using a substantially fixed length connector  334  such as a rod or the like. In the illustrated construction, the connector  334  includes a pair of flared ends  335 . These flared ends may slidingly fit in dovetailed relation within opposing slots extending at least partially along the length of the support pillars  326 ,  328 . As will be appreciated, such a structure may provide a substantially fixed span between first support pillar  226  and the second support pillar  228 . Moreover, rods of different length may be interchanged. Thus, the distance between the targets may be adjusted as desired. 
     In accordance with another exemplary construction consistent with this disclosure, a target assembly of substantially unitary construction may be provided. By way of example only, and not limitation,  FIG. 8  illustrates an exemplary target assembly  420  of unitary construction wherein elements corresponding to those previously described are designated by like reference numerals within a 400 series. In the exemplary construction illustrated in  FIG. 7 , the first support pillar  426  and the second support pillar  428  form parts of a unitary body  470  such as may be formed by techniques such as molding, machining, electrical discharge machining “EDM” or the like. As shown, the unitary construction target assembly  420  provides a double lobed face structure  444 , which may be used in contacting relation with a curved surface during evaluation. First support pillar  426  and Second support pillar  428  are adjoined by a reduced width intermediate neck portion  472 . As illustrated, this intermediate neck portion  472  may have a substantially concave edge profile defining a saddle or detent between the lobes at the face structure  444  thereby yielding a unitary body structure of substantially dog-bone shape. Of course, the illustrated unitary construction is exemplary only and any number of other constructions may likewise be utilized as may be desired. 
     In practice, the use of constructions having substantial spans between support pillars as illustrated in  FIG. 5 ,  FIG. 6 , and  FIG. 7 , may find particular application in measurement of surfaces having relatively modest degrees of curvature. In this regard, as the curvature of the surface being evaluated approaches a flat plane, it may be desirable to increase the distance between support pillars. Likewise, as the degree of curvature at the measurement surface increases, it may be of benefit to reduce or eliminate any span between the support pillars. In such environments of use, configurations such as those illustrated in  FIGS. 2 and 8  may be useful. 
     INDUSTRIAL APPLICABILITY 
     The industrial applicability of the photogrammetric target assembly described herein will be readily appreciated from the foregoing discussion. The present disclosure is applicable to target assemblies adapted for use in photogrammetric evaluation of surfaces to determine and/or confirm surface contour characteristics. 
     In practice, a target assembly incorporating a pair of reflective targets disposed at terminal show surfaces of operatively connected and laterally-spaced support pillars is attached to a portion of a curved evaluation surface. The spaced support pillars may define a double lobed face structure to maintain contact with the curved evaluation surface. An imaging device is utilized to measure the positions of the reflective targets. The target assembly and associated measurement practices may be used in the evaluation of substantially convex curved surfaces such as the outer wall of a shaft or the like. The target assembly and associated measurement practices may also be used in the valuation of substantially concave curved surfaces such as the boundary wall of a borehole or the like. Multiple target assemblies may be used concurrently across the inner wall and outer wall of annular structures such as pipes and hollow shafts to evaluate conformance with sizing specifications. 
     It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to examples herein are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure or claims more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the claims entirely unless otherwise indicated. 
     Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. 
     Accordingly, this disclosure contemplates the inclusion of all modifications and equivalents of the subject matter recited in the appended claims as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is contemplated unless otherwise indicated herein or otherwise clearly contradicted by context.