Abstract:
Disclosed in the present disclosure is a phased array system configured to ultrasonically inspect test targets complex surfaces while employing the surface profiling capability of phased-array linear and sectorial scans. Adaptive focusing is employed for inspecting the test target by using customized apertures according to the surface profiles to generate a plurality of beams that are evenly and thoroughly spaced along a scan line inside the test target.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to non-destructive testing and inspection systems (NDT/NDI) and more particularly to an improvement applied to ultrasonic phased array systems that allows adaptive focusing for inspecting target with complex shaped surfaces. 
       BACKGROUND OF THE INVENTION 
       [0002]    Test targets with curved, wavy or irregular surfaces have long been a challenge for ultrasonic testing. Different paths have been exploited and explored to resolve problems in this challenge. 
         [0003]    One existing effort seen in U.S. Pat. No. 6,424,597 involves using flexible transducers that, to a certain extent, offset the geometric variations to optimize the acoustic coupling and integrate a profile-meter. The profile-meter makes it possible to offset, using delay laws, the aberrations that the ultrasonic beam may undergo when it passes through a complex interface. However, the transducers of this type are put directly in contact with a target piece to be monitored. This leads to the existence of a non-inspectable range of several millimeters (a “dead zone”) under the surface of the piece. To resolve this problem, US 2011/0032800 uses a method to connect a delay line to each element of the flexible transducer. However such solution introduces a detrimental drawback of significantly reducing the transducer&#39;s flexibility. These transducers are also not suitable to perform inspection with large incline angles of the refracted beam. These transducers are also typically quite complicated mechanically and can be quite costly, limiting their acceptance by the general market. 
         [0004]    Improvements to the flexible transducer concept are being explored in US 2011/0032800, in which a rigid phased-array transducer is used in conjunction with a flexible wedge and a profile-meter to provide focal laws for inspecting a part with complex geometry. However, this solution significantly complicates the inspection and requires additional costly hardware. 
         [0005]    Other existing methods have also been explored that do not require complicated profile-meter hardware. Using phased-array ultrasound, it is possible to compensate in many cases for known surface geometries by adjusting the time delays used in transmission and reception. Focal law calculators are commercially available that allow phased-array ultrasonic beams to be designed for simple regular surface geometries. These techniques typically use a prior knowledge of a surface profile to calculate delay law parameters as a function of the position of the probe on the target. These techniques are beneficial in the case of a slightly irregular surface, but their usefulness becomes very limited when the surface is warped due to positioning errors of the transducers and lack of knowledge of the surface&#39;s profile. 
         [0006]    In U.S. Pat. No. 7,823,454, a method is used in a phased-array probe to ultrasonically define the surface profile of a target. This technique uses a full-matrix-capture technology to process ultrasound data in order to obtain the profile of the surface of the test target and then to inspect the volume of a target by processing the data to compensate for surface irregularities using focal laws corrected for the surface profile. Although the full-matrix capture technology can provide some degrees of advantages over traditional pulse-echo phased array, it presents the disadvantages of requiring substantial data storage and processing requirements. 
         [0007]    US patent publications US 2011/0120223 and US 2007/0056373 also exploit similar methods using phased-array ultrasound to determine the surface profile of a test target acoustically and perform inspections with adaptive phased-array focal laws. In these efforts, the entire phased-array probe is used to provide a single sound beam and as such, these attempts do not appear to use the full potential of phased-array system; notably the imaging offered by sectorial and linear scans which are comprised of multiple sound beams can contain volumetric information. 
         [0008]    Sectorial and linear scans provide imaging by sonicating a larger region within a test target than a single sound beam can provide and therefore can display the acoustic information obtained from the plurality of sound beams volumetrically. 
         [0009]    With linear scans used in PA, the same focal laws are applied for successive active apertures of a phased-array probe. Focal laws are time delays used when pulsing a plurality of elements of a phased array probe in an active aperture to form a sound beam with a predetermined focal position and steering angle (i.e. angle between sound beam and the probe surface). For test targets with simple geometries, refraction at the test target planar surface provides the same consistent refraction angle (i.e. the angle between sound beam and the target&#39;s surface) for all sound beams in a linear scan. 
         [0010]    However, this standard definition of linear scan cannot be adequately applied to obtain representative volumetric inspections of complex surface targets. As depicted in  FIGS. 1   a  and  1   b , a simple geometry is compared to a complex geometry when the same linear scan is applied. In  FIG. 1   b , the refracted sound beams are not evenly distributed within the target, creating substantial dead-zones in the inspection coverage. Some beams do not even enter into the target due to their high incidence angle on the target complex surface. 
         [0011]    With sectorial scans, the active aperture is fixed and focal laws are successively applied to incrementally produce varying steering angles. For a simple surface geometry, this translates into evenly distributed refracted beams at varying refraction angles. However, as with linear scans, it is not possible by using existing sectorial scan techniques to produce evenly distributed refracted beams within test target with complex surface. 
       SUMMARY OF THE INVENTION 
       [0012]    Accordingly, it is an objective of the present disclosure to provide a method of ultrasonically inspecting test targets having complex surfaces while employing the imaging capability of phased-array linear and sectorial scans. 
         [0013]    It is further an object of the present disclosure to define adaptive focal laws for providing linear scan results of the interior of a test target by employing customized apertures to generate a plurality of beams that are evenly spaced along a scan line and have the same refraction angle inside the test target. 
         [0014]    It is further an object of the present disclosure to define adaptive focal laws for providing sectorial type scan results of the interior of a test object by employing customized apertures to generate a plurality of beams that all pass through a common point and are angularly evenly spaced with respect to refraction angle inside the test target. 
         [0015]    Another objective of the present disclosure is to provide for the use of a typical phased-array probe to perform adaptive focusing in order to inspect targets with complex surfaces. 
         [0016]    Yet another objective of the present invention is to provide methods for measuring the surface profile of a complex target such as a weld cap using phased array ultrasonic testing. 
         [0017]    The invention disclosed herein aims to resolve the aforementioned drawbacks related to the known arts for ultrasonically inspecting a target with a wavy or uneven surface. A typical phased-array probe is operated with a substantial fluid layer such as water between the array transducer and the test target surface. The fluid layer is maintained by immersing the target in liquid or by using a captive couplant column between the probe and the target surface. The surface profile of the target is measured acoustically for a given probe position. Adaptive phased-array focal laws for both sectorial and linear scans are defined and re-emitted based on improved electronic scan concepts and the measured surface profile. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIGS. 1   a  and  1   b  present schematic diagrams showing prior art of a linear scan applied respectively on simple and complex surface test targets. 
           [0019]      FIG. 2  is a schematic diagram showing the presently disclosed phased-array adaptive focusing system. 
           [0020]      FIG. 2   a  is a schematic diagram showing an alternative embodiment of the presently disclosed phased-array adaptive focusing system. 
           [0021]      FIGS. 3   a ,  3   b ,  3   f  form a group of schematic diagrams showing an example of a multi-group focal laws arrangement used for surface profiling. 
           [0022]      FIG. 4  is a schematic diagram showing an example of ray-tracing used to determine the center-of-aperture on a phased-array transducer for the case of a sectorial angle beam scan using true depth focusing. 
           [0023]      FIG. 5  is a schematic diagram showing an example of ray-tracing used to determine the center-of-aperture on a phased-array transducer for the case of an angle beam linear scan using true depth focusing. 
           [0024]      FIG. 6  is a functional block diagram showing the procedure of PA inspections with adaptive focusing deployed according to presently disclosed method. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Referring to  FIG. 2 , an adaptive phased-array inspection system  3  according to a preferred embodiment of the present invention is comprised of a phased-array (PA) probe  1 , an acquisition unit  2  and a data processing and display unit  16 . Data processing and display unit  16  can be an existing PA system. A test object or target  4  featuring a complex inspection surface  5  that takes the form of weld cap  6  is herein used as an exemplary test target since it closely pertains to the problem that the present disclosure deals with. Albeit the complex nature of surface  5 , ultrasound beams are required to pass through the surface in order to inspect within the volume of the target  4 . It should be noted that PA probe  1  can interchangeably be one of a plurality of phased array probes compatible with system  3 . Probe  1  is coupled to test target  4  via a layer of substantial amount of fluid by either immersing the target and transducer or by using a captive water column between the transducer and target surface (not shown). 
         [0026]    Adaptive phased-array inspection system  3  (later as “adaptive system  3 ”) further embodies a surface profile module  10  and an adaptive focusing module  14 . Surface profile module  10  receives information from data acquisition unit  2 , produces a surface profile pertaining to the complex surface  5 . Adaptive focusing module  14  then employs an adaptive focusing process, instructing processing and display unit  16  to perform adaptive focusing. 
         [0027]    It should be appreciated that acquisition unit  2  and processing and display unit  3  can alternatively be assembled integrally in a more portable version of PA system  3 , the embodiment of which is within the scope of the present disclosure. 
         [0028]    It can be understood that the adaptive system shown in  FIG. 2  comprises novel components of profile module  10  and adaptive focusing module  14 , which can be added onto an existing PA system  3  (processing and display unit). Alternatively as shown in  FIG. 2A , novel components of profile module  10  and adaptive focusing module  14  can be deployed directly within an integral part of a new PA system  3 , together with conventional existing phased-array components, collectively as  15 . It should be appreciated that the configurations shown in both  FIG. 2  and  FIG. 2A , namely using the novel components as add-on portions to an existing PA system, or employing such novel components as an integral part of a newly designed PA system, are within the scope of the present disclosure. 
         [0029]    Reference is now made to  FIGS. 3   a ,  3   b ,  3   f  which exhibit more details on how a surface profile is provided with surface profile module  10 . The surface profile of target  4  is measured acoustically by first acquiring multiple phased-array linear scans using PA probe  1 . This represents one of the novel aspects of the present invention, since conventionally phased operations directly engage into inspection, assuming the surface of the test object to be flat. As shown in  FIG. 3   a , PA transducer  1  is not substantially parallel to the nominal target surface reference  22 . According to the preferred embodiment of the invention, distance D and angle α between probe  1  and the surface  5  are known except for the region in the vicinity of the weld cap  6  (in  FIG. 2 ). The surface profile of target  4  can be determined acoustically by profile module  10  according to data acquired by acquisition unit  2  from multiple phased-array linear scans with at least two steering angles (i.e. angle between PA probe active surface  24  and acoustic beams). As depicted in  FIG. 3   a , a first linear scan  11  is performed with the acoustic beams directed substantially perpendicular to the expected nominal target surface orientation. Additionally, as depicted in  FIG. 3   c , a second linear scan  12  is performed without a steering angle such that the acoustic beams are basically perpendicular to the PA probe active surface  24 . Advantageously, the plurality of beam angles employed for surface profiling provides more appropriate surface profiling of complex geometries. 
         [0030]    The acoustic information obtained by this plurality of linear scans as shown in  FIGS. 3   a - 3   f  can be processed to profile the entire surface of the target through which inspection beams traverse. For example as shown on  FIG. 3   b , linear scan  11  provides bases for profiling about surface profile sections marked as  34  and  35 , as the acoustic beams are more or less perpendicular to these surface sections. Linear scan  12  provides profiling of surface section  36  of  FIG. 3   d  for the same reasons. As shown in  FIGS. 3   e  and  3   f , combing the profiling information from this plurality of linear scans allows for profiling the entire relevant surface profile  37 . Dashed line  32  shows the actual surface profile in this example. 
         [0031]    With the knowledge of the target complex surface distribution with respect to probe  1  provided by the surface profiling method described above, adaptive focal laws can be further performed by the adaptive focusing module as described below. 
         [0032]    Focal laws for simple geometries are typically defined by a user selected parameters such as focal depth and beam refraction angle for linear scans or angles for sectorial scans. Beam spacing is also used to define the overall scan resolution. This approach in the conventional practice is adopted herein. In this embodiment, focal depth, beam refraction angle and beam spacing constitute the principle beam parameters. These beam parameters are defined with respect to the nominal target surface reference  22  shown in  FIGS. 3   a - 3   f . 
         [0033]    Reference is now primarily made to  FIG. 4 , with continued reference to previous figures to describe the principle and scope of adaptive focusing devised by the present disclosure. In an exemplary case adaptive focusing is applied by module  14  with focal law definition for sectorial scans corresponding to measured surface profile  37 . In an adaptive sectorial scan according to the exemplary embodiment, beam intersection point  50  is situated vertically on target surface reference  22  and is defined horizontally by the user or by some other means. For instance, in the case of weld bevel inspection, intersection point  50  could be chosen as to ensure complete coverage of the bevel line. It could also be defined simply by extending a perpendicular line from plane  22  to the middle of the phased-array. It should be noted that this represents one of the novel aspects of the present invention as conventional phased-array sectorial scans are characterized by a beam intersection point on the probe active surface  24 . From beam intersection point  50 , a plurality of beams  52 ,  53 ,  54  and  55  can be extended according to beam parameters such as refraction angle and beam angular spacing. Beam refraction angles are defined based on plane  57  which is perpendicular to target surface reference  22  in such a fashion that beam  52  is defined by refraction angle  520 , beam  53  is defined by refraction angle  530  and so on. Beam spacing for sectorial scans is defined as the angular gap between successive beams. The last critical beam parameter is focal depth referred as  23  in  FIG. 4 , which is defined as a plane parallel to target surface reference  22  offset vertically by distance  51 . Focal points  521 ,  531 ,  541 , and  551  are defined at the intersection of beams  52 ,  53 ,  54  and  55 , respectively, and focal depth  23 . 
         [0034]    Continuing with  FIG. 4 , the geometrical extension of beams  52 ,  53 ,  54  and  55  from their respective focal points through beam intersection point  50  towards probe active surface  24  is used to define the aperture position along the phased array that is the most appropriate for a given beam. Upon intersecting measured surface profile  37 , commonly known Snell&#39;s law is used to calculate the beam incident angle prior to refraction according to refraction angles  520 ,  530 ,  540  and  550  and the known sound velocities of target  4  and the fluid coupling layer. For example, the extension of beam  52  intersects with probe element  62  on probe active surface  24  whereas the extension of beam  55  intersects with probe element  65  on probe active surface  24 . Elements  62  and  65  are then defined as the center of aperture for generating phased-array beams  52  and  55  focusing at focal points  521  and  551 , respectively. Once an optimal center of aperture is selected according to the above method for a given focal point, conventional phased-array focal law calculation methods can be deployed to calculate pulsing delays for all of the phased-array probe elements that constitute a given aperture, contributing to a given focal law. In this regards, a focal law is a precise combination of element delays in a given aperture for focusing at a precise focal point according to the respective surface profile. 
         [0035]    Reference now is made primarily to  FIG. 5 , with continued reference made to previous figures.  FIG. 5  illustrates a process which can be devised in an alternative embodiment of focusing module  14 , using linear scans to achieve the adaptive focusing corresponding to surface profile  37 . Similar to the previous example shown in  FIG. 4 , PA probe  1  is not substantially parallel to nominal target surface reference  22  and the distance D and angle α between probe  1  and the surface  5  are known except for the region in the vicinity of the weld cap  6 . 
         [0036]    After beam parameters are defined by the user, a plurality of focal points  720 ,  730 ,  740  and  750  can be defined. In this embodiment, all focal points are defined on the horizontal plane associated with focal depth  23  at a distance  51  below the target surface reference  22 . From each of these focal points, a beam is traced starting from the focal points towards the nominal target surface reference  22  with an orientation parallel to refraction angle  70  defined related to a plane perpendicular to target surface reference  22 . For example, from focal point  720 , beam  72  is traced with angle  70  towards reference plane  22 . 
         [0037]    Continuing with  FIG. 5 , with the surface profile found by module  10 , the geometrical extensions of beams  72 ,  73 ,  74  and  75  from their respective focal points along an orientation parallel to refraction angle  70  towards probe active surface  24  are used to define the respective phased-array probe aperture that is the most appropriate for a given beam. Upon intersecting measured surface profile  37 , commonly known Snell&#39;s law is used to calculate the beam incident angle prior to refraction according to refraction angle  70  and the known sound velocities of target  4  and the fluid coupling layer. For example, the extension of beam  72  intersects with probe element  721  on probe active surface  24  whereas the extension of beam  75  intersects with probe element  751  on probe active surface  24 . Elements  721  and  751  are subsequently defined as the center of aperture for generating phased-array beams  72  and  75  focusing at focal points  720  and  750  respectively. Similarly, once an optimal aperture center for all the intended focusing points has been selected, conventional phased-array focal law calculation methods can be deployed to calculate pulsing delays for all of the phased-array probe elements in a given aperture, contributing to a given focal law. In this regards, a focal law is a precise combination of element delays in a given aperture for focusing a precise focal point according to the respective surface profile. 
         [0038]    It should be noted that the linear scan or sectorial scan can be also herein referred to as an electronic scan. 
         [0039]    Referring primarily now to  FIG. 6  and continuingly to previous figures, the surface profiling and adaptive focusing methods as aforementioned are described in a flowchart diagram. In a first step  80 , the beam parameters are defined, adopting conventional practice. These would typically be defined by the user. These parameters include but are not necessarily limited to: material acoustic velocity, delay-line parameters, inspection scan type (linear or sectorial), refraction angle or angles, focusing type and distance and aperture size, beam spacing. Delay-line parameters can include delay-line acoustic velocity, height and nominal angle between transducer active surface and target surface (if known). 
         [0040]    In step  81 , the surface profile of the target is obtained by executing surface profile module  10 , which executes a sequence of two sub-steps,  81   a  and  81   b.  In step  81   a,  multiple runs of phased-array acquisition are performed according to the method described in group  FIGS. 3 . Step  81   a  includes multiple runs of phased-array acquisitions used for acquiring acoustic data for the intent of surface profiling and would typically include two or more combinations of sectorial and/or linear scans at different steering angles. In step  81   b,  the profile module calculates the complex surface profile distribution according to the data acquired in  81   a.    
         [0041]    With the surface profile determined in the abovementioned step  81 , adaptive focal laws are calculated for a given scan position in step  82 , which is executed by adaptive focusing module  14 . Step  82  comprises sub-step  82   a  in which an ultrasonic ray is traced from the focal point to the probe active surface by applying Snell&#39;s law at the target surface interface. In the case of a sectorial scan, all rays would intersect at a pre-determined position  50  shown in  FIG. 4 . Sub-step  82   b  comprises defining the center-of-aperture of the beams as the position on the phased-array transducer active surface where the ray impinges and sub-step  82   c  comprises calculating focal law delays for an aperture of a given number of elements centered at the center-of-aperture. Steps  82   a,    82   b  and  82   c  are repeated for all beams in the scan. Method and process associated with  FIGS. 4 and 5  should be employed in implementing details of step  82 . 
         [0042]    In step  83 , the same phased-array transducer is used to acquire acoustic data for all beams in a scan by using the adaptive focal laws calculated in step  82  relative to the surface profile determined in step  81 . In step  84 , the acquired acoustic data is stored and typically displayed to the user. 
         [0043]    After all of the focal laws are acquired for all beams in a scan for a given probe position, a typical scan would include moving or incrementing the probe to a different position on the target part and the above mentioned steps from  81  to  83  are repeated in order to profile the target surface, calculate new adaptive focal laws and acquire acoustic data with the adaptive focal laws . 
         [0044]    It should be noted that the steps  81 ,  82 ,  83  and  84  could form a complete scan at one inspection location. For an example of weld inspection, at one specific weld axial location, the system can be operated to execute steps  81 ,  82 ,  83  and  84  to achieve one scan sequence for inspection of the corresponding weld axial location. When the probe is moved onto the subsequent axial location, another round of steps  81 ,  82 ,  83  and  84  can be repeated. 
         [0045]    However, the present disclosure is not restricted to such scanning routine. Alternatively, especially when the weld surface is not expected to change dramatically, the rate of the execution of routine  81  might be chosen to be slower than the rate of scan. In another words, the surface profile does not have to be updated for each scan sequence. It can be alternatively defined by the user to be updated, for example, once every two or five, or 10 scans sequence, depending on the uniformity and consistency of the weld are perceived to be. 
         [0046]    Although the present invention has been described in relation to particular exemplary embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention not be limited by the specific disclosure. 
         [0047]    Although the above descriptions have been shown to apply to a phased-array transducer not substantially parallel to nominal target surface reference  22 , it must be recognized that the scope of this invention is intended to cover alternative relative positions of phased-array transducers and target surface as well as alternative refraction angles. Notably, the phased-array transducer could be positioned substantially parallel to the nominal target surface reference. This invention would also apply to using phased-array transducers that are not substantially flat. 
         [0048]    Furthermore, although the preferred embodiment described two or more linear scans to be used for surface profiling, it must be recognized that any combination of electropnic scans can be used to this effect. 
         [0049]    It must also be recognized that although true depth focusing is described herein, this invention is not specific with respect to the focusing type. As such, focusing alternatives such half-path and custom plane projections are within the scope of this invention. 
         [0050]    It should also be recognized that the electronic scan beam definitions described herein would apply to other similar phased-array acquisition methods such as full-matrix capture. 
         [0051]    Although an immersion type delay-line is described herein, it must be recognized that alternative adaptable coupling methods such as soft conformable polymeric materials are compatible with the teachings herein, which would not affect the scope of the present invention.