Abstract:
A calibration method for calibrating a phased array probe that is used for testing girth welds for defects. The method utilizes a calibration device on which is defined a series of reflectors that correspond to a series of target zones. The phased array probe is placed via a wedge relative to the calibration device and the phased array probe is configured with an initial set of acoustic parameters which define at least a transmitting aperture, a receiving aperture and a beam steering angle. Using a Full Matrix Capture (FMC) acquisition process and a ray-tracing module, the values of the initial set of acoustic parameters are optimized to evolve a final set of acoustic parameters which the phased array probe utilizes for testing actual devices for weld defects.

Description:
FIELD OF THE INVENTION 
     This invention relates to non-destructive testing and inspection systems (NDT/NDI) and more particularly to automation of calibration for a girth weld pipe inspection system using phased array ultrasound technology (PAUT). The invention also relates the use of Full Matrix Capture (FMC) acquisition schemes to predict PAUT parameters through an automated calibration process according to the present disclosure. 
     BACKGROUND OF THE INVENTION 
     The use of PAUT for the inspection of pipeline girth weld has been described in various publications such as “Pipeline Girth Weld Inspection using Ultrasonic Phased Arrays” (by Michael Moles, Noel Dube, Simon Labbé, Ed Ginzel). The practice is also incorporated in industrial standard practice such as ASTM E-1961-11 “Standard Practice for Mechanized Ultrasonic Testing of Girth Welds Using Zonal Discrimination with Focused Search Units”. 
     Practically, this inspection is based on the zone discrimination technique which involves the definition of and calibration of multiple beams to obtain desired detection performance on a set of pre-defined artificial defects. The PAUT beams use a pitch-catch configuration which means each beam is impacted by the definition of a transmitter and a receiver. The calibration itself is then relatively complex and time consuming because of the large number of possible settings for each beam. 
     Furthermore, since the required configuration is dependent on the specific weld geometry to be inspected, a new calibration is required every time changes are made to the inspection system. For calibration of pitch-catch inspection with the zone discrimination technique, an inspector needs to calibrate for each zone of interest in a calibration block to be sure to meet inspection criteria for all potential defects. More specifically, for each zone, the inspector needs to align the probe on the relevant calibration reflector, to adjust the steering angle, the aperture (position and size) and focalization of the transmitter, the aperture of the receiver (position and size), focalization and the gain of the receiver to perform calibration. With today&#39;s methods, due to the plurality of calibration reflectors, a typical calibration procedure takes several hours to perform and requires a high degree of expertise by the operator. 
     A side effect of the current level of complexity of the calibration is a compromise in calibration requirement between realistic calibration time and performance. 
     PAUT Girth weld inspection is the most common example of the use of pitch-catch PAUT, but it must be understood that the same or similar limitations are found in most systems that rely on pitch-catch PAUT. 
     So, there is therefore a need for a method to automate the calibration process of pitch-catch PAUT inspection in order to reduce the calibration time and the dependency on inspector&#39;s skills. 
     There&#39;s also a need for a method that would meet tighter calibration tolerance than current PAUT inspection methods in order to improve detection performances and reproducibility. 
     SUMMARY OF THE INVENTION 
     This invention uses FMC acquisition technique to find the best acoustic configuration of PAUT beam to reach calibration requirements within each zone to be covered by the inspection; each zone being associated to an artificial reflector on a known reference sample. The FMC acquisition technique is known to include all physical information required to build a PAUT signal in post processing. Therefore a single FMC acquisition can be used to evaluate the inspection result that would have been obtained using any PAUT inspection. 
     An optimization method is disclosed to define the set of acoustic parameters required for generating and receiving the PAUT beams that would provide the best performances for each target reflector based on the set of FMC data. This method uses operator inputs such as probe position and inspected component definition to define theoretical beam configurations. From this theoretical configuration and including tolerances representative of the application for each parameter involved, the optimization process automatically defines the best calibration values to reach the application requirements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view representation of the pitch-catch PAUT inspection configuration associated with girth weld inspection. 
         FIG. 2  is a top view representation of the pitch-catch PAUT inspection configuration associated with girth weld inspection. 
         FIGS. 3 a  and 3 b    are the representation of a first PAUT aperture, the associated response on a given set of reference flaws and illustrating how the aperture affects the detection of previous and next target. 
         FIGS. 4 a  and 4 b    are the representation of a second PAUT aperture, the associated response on a given set of reference flaws and illustrating how the aperture affects the detection of previous and next target. 
         FIG. 5  is a general flow chart exhibiting the automatic calibration process according to the present disclosure for a girth inspection using PAUT and FMC scheme. 
         FIG. 6  is a flow chart describing specifically the automated calibration process. 
         FIG. 7  is a flow chart describing a particular aspect of the automated calibration process which is the iterative generation of test parameter within defined range. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows a phased array probe (PAP)  3000 , of the type used for phased array ultrasound testing (PAUT). A side view representative of a PAUT pitch-catch inspection configuration for detecting a set of planar flaws in a girth weld is illustrated on  FIG. 1 . The ray-tracing representation of the PAUT beam  3070  is a simplified illustration of the acoustic energy path associated to the generation of an acoustic wave by a first aperture  3030  with a first set of delays and received by the second aperture  3020  with a second set of delays. Generation and reception apertures are defined by the position and number of PAUT element  100  included in the so called aperture. As for the delays, they are typically being used to change steering angle  3050  for the acoustic wave generation, the steering angle  3110  for the acoustic wave reception of the beam and the focalization distance of the acoustic wave. 
     Also represented on the  FIG. 1  is a set of reflectors  217 ,  218 ,  219 ,  220 ,  221 ,  222 ,  223  and  224 . Those reflectors will be referred to as the TARGET in this document; they represent a set of artificial reflectors machined on the calibration block  2320 . A number of beams corresponding to the number of targets are generated, each beam being associated to a specific TARGET. For example, on  FIG. 1 , beam  3060  is associated to TARGET  219 , beam  3070  is associated to TARGET  220 , and beam  3080  is associated to TARGET  221 . Eventually, the objective of the calibration is to define the set of acoustic parameters (aperture width and position, steering angle and focalization for transmitter and receiver) required for each of the beams to obtain an optimized detection of the each associated TARGET for a real inspection configuration. 
     As illustrated on  FIG. 2 , which is a top view representation of the inspection, the various TARGET are positioned at different scan position (SP) on the calibration bloc in order to decouple the detection of each TARGET relative to the beams. The SP of each TARGET on the block is a known parameter for the inspection although this knowledge is only accurate to within certain tolerance. In this document, the various TARGET are sequentially distributed in the calibration block (i.e. the previous an next TARGET to  220  relative to the scan position on  FIG. 2  are also the previous and next TARGET to  220  on the side view of  FIG. 1 ), this representation is only to simplify the discussion and is not a limitation of the method. Reference made to previous and next TARGET for the method of the invention must be understood from the side view representation of  FIG. 1 . 
     Another aspect to consider for the calibration is the detection level obtained by a given beam  3070  on previous  221  and next  219  TARGET. Typically, the objective is to have about 10 dB less amplitude on previous and next TARGET relative to the current TARGET, tolerances over and above this value are then provided for defining the calibration success. Aperture width and focalization are the key acoustic parameters for reaching this objective. The  FIGS. 3 a , 3 b    and  FIGS. 4 a  and 4 b    are simplified representation of the effect of varying transmitter aperture width  3030   a  to  3030   b  on the signal amplitude at SP 219 , SP 220  and SP 221 . With the larger aperture  3030   a , the peak amplitudes P 219  and P 221  obtained at SP 219  and SP 221  are approximately 12 dB below the peak amplitude P 220  of the TARGET defect at SP 220 . With the narrow aperture  3030   b , the peak amplitudes P 219  and P 221  obtained at SP 219  and SP 221  are approximately 6 dB below the peak amplitude P 220  of the TARGET defect at SP 220 . Similar effect would be obtained by varying the receiver aperture  3020 . As for the focalization, it must be understood that difference between the current and next/previous TARGET will be maximum when the transmitter and receiver focalization is precisely on the TARGET. So, focalization is another parameter to consider for precisely achieving a 10 dB between current and next/previous TARGET. 
     Now looking at  FIG. 5  which provides a global view of the calibration process including the automated aspects covered by the invention, it is observed that the two sources of inputs for the automatic calibration  1040  are the ray-tracing module  5020  and the set of full matrix capture (FMC) data  1030  acquired on each TARGET. The FMC data is acquired at a relevant scan position for each TARGET (i.e. including known mechanical tolerances). Technically, if those FMC data includes all elements  100  of the transmitter and receiver aperture for a given TARGET, any phase array (PAUT) beam can be precisely simulated from those data in order to conduct the automated calibration process  1060  for this TARGET. 
     The ray tracing module  5020  uses known information  1020  on the probe  3010  and wedge  3040  assembly  3000 . That information typically includes the size and pitch of the probe element  100  as well as the precise localization of these elements relative to the part upper surface  2150 . The ray tracing module  5020  also uses information pertaining to the part description  1025  as well as the calibration bloc description for zonal break-down. That information typically includes the inspected part thickness  2155  and the details of the weld  2310  geometry. Another information required by the ray-tracing module  5020  is the position  3120  of the probe assembly  3000  relative to the weld centerline  2000 . The ray-tracing module is then able to theoretically predict all beam configurations required for detecting each and every TARGET. The calibration process  1060  will then use this theoretical information to restrain the calibration in relation to known practical tolerances. 
     Still looking at  FIG. 5 , we illustrate the fact that the calibration  1060  must successfully achieve the calibration on all TARGET (step  1070 ) in order to complete the process and provide a set of calibration values in  1090  allowing the remainder of the normal calibration process  1100  to be conducted (including GAIN setting and a validation scan with PAUT). Typically, the calibration will not be met for all targets if the probe is too close or too far from the weld. In this case, the required aperture position and length can&#39;t be achieved with the actual probe position  3120  and it may be possible to indicate to the user how (in which direction) to move the probe from the previous failed calibration  1060  in order to make a new calibration scan. 
     Now looking at the details of  1060  in  FIG. 6 , we start from the two inputs  1030  and  5020 . Inspection results for the current, next and previous TARGET are generated in  6100  by selecting test parameters within the range defined by the ray-tracing and known tolerances for the inspection. Results generated by  6100  are analyzed in  5140  in order to find the configuration that provides a maximal response on the current TARGET. A feedback from  5140  to  6100  makes it possible to orient the parameters search through the use of a Tabu search algorithm (which is part of  5140 ). Short term Tabu list values (i.e. list of values to avoid) generated during this search expire after step  5140  completion. Tabu search algorithms are well known in the art to perform such task. 
     Results of step  5140  are the calibration values  5130 , which are then used for the previous and next TARGET analysis. A calibration value  5130  includes all relevant acoustic parameter such as transmitter and receiver aperture definition, focalization and steering angle. Step  5180  and  5190  account for the need of this alternate test for the application and the specific TARGET being evaluated. For cases where previous TARGET  221  validation is required, the FMC data corresponding to the acquisition of the previous TARGET  221  at scan position SP 221  are processed with calibration values  5130  by finding the maximum amplitude recorded P 221  within the scan position tolerances of SP 221 . Obviously, if the resulting amplitude is within the expected range the process goes on for an equivalent validation on the next target if needed. In cases where the resulting amplitude isn&#39;t in the desired range, the current calibration values are put in the long term Tabu list of the Tabu search algorithm. 
     A further validation is a search for conditions that prevent a solution to be found within the defined tolerance and with the current FMC data set, the extreme case being that all possible combination of parameters have been evaluated without success. This validation is represented by block  5135 , a specific condition can lead to a decision that the probe assembly  3000  must be moved in order to find a solution or that tolerance must be increased by monitoring the evolution of the long term Tabu list and the associated trend on the step  5250  and  5260  validation results. Once calibration values are found that satisfies both  5250  and  5260 , the calibration of the beam for the current TARGET is completed and the calibration values are saved for further steps. 
     Now looking at  FIG. 7 , we have a detailed overview of the various sub-steps involved in  6100 . Block  7010  represents the allowed scan position range while other blocks  7020 ,  7030 ,  7040 ,  7050  and  7060  represent acceptable range for the various acoustic parameters corresponding to the calibration values themselves. Specific parameters to be used for the processing of FMC data within  7200  are represented by  7110 ,  7120 ,  7130 ,  7140 ,  7150  and  7160 . The specific selection of those parameters is oriented by the maximization algorithm  5140 . 
     It must be understood that although the automated calibration process is described here for PAUT girth weld inspection, it could also be used for other pitch-catch PAUT inspections as long as the calibration process is based on the use of PAUT beams to be set on a calibration block with known acoustic reflectors. 
     It must also be understood that the calibration values provided as a result of the invention in  1090  can be used either for further inspection with PAUT or/and FMC acquisition. 
     It should be noted the FMC is often known to deploy phased array operations involving all elements in a matrix probe. However, with the same operational principle used by the conventionally known FMC, the techniques involved in the present disclosure can optionally include those operations that only use a portion of the matrix element. This applies to both transmitting and receiving side of operations, and to the situations when the number of elements on the transmitting side differs from that on the receiving side. The variation depends on how the apertures are selected and arranged, all of which should be understood by those skilled in the art and are all within the scope of the present disclosure. 
     Although the present invention has been described in relation to particular 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 be limited not by the specific disclosure herein, but only by the appended claims.