12 A High-Speed Dual-Stage Ultrasonic Guided Wave System for Localization and Characterization of Defects 131 Fig. 12.6 Example of generated heatmap 12.3.5.1 Scan Area Selection The localization results obtained in Stage 1 are used to guide the laser to an area to scan. Reducing the size of the scan area will reduce the amount of time needed to image the defect and provide a higher resolution scan. We would like to choose the smallest scan area possible while still being reasonably confident that our scan area includes the damage. We examine the range of local maximum likelihood estimates produced in Stage 1, and define the scan area to be a rectangle that includes all pixels whose likelihood estimates exceed a threshold. After some experimentation our threshold was chosen as 90 %. In future work, a more rigorous statistical analysis could be used to determine scan area. 12.3.5.2 Active Transducer Selection In order to produce the steady-state excitation necessary for a high-fidelity laser scan, we take advantage of our distributed array of transducers by selecting the transducers nearest to the estimated damage location. The distributed nature of our transducer array allows us to route energy close to the scan area. Experimental testing has shown that the best scan images are produced when the area of suspected damage is surrounded with a transducer on each side. Intuitively this makes sense because we are able to route energy around structural features that may damp out waves coming from one direction. Thus, we look for a subset of transducers that are near the damage location and that effectively surround it. Our set of active transducers will include four transducers. It is possible that one or more of them will be within the scan region, while others may be outside the scan region. We begin by selecting all transducers contained within the scan bounds. These transducers belong to set I. For each sensor inside the scan bounds we calculate the closest side to the transducer by finding the side that minimizes the distance to the midpoint of the side. There are then S remaining sides where S can be any integer in the range [0,4]. The set A contains all the transducers outside the scan area. The set of transducers outside the scan area to energize, O, are chosen as those that minimize the sum of the distances to the remaining sides O Dargmin i2 A 0 @ S XjD1 dij1 A (12.9)
RkJQdWJsaXNoZXIy MTMzNzEzMQ==