THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

16.7 Basic Instrumentation: Scanning Methods 473SYNCGENERATORPULSERPULSELIMITERAMPLIFIERDISPLAYTRANSDUCERCOMPUTERWORKPIECEBEAMBACK FACEA-ScanFigure 16.9. Block diagram of a “generic ultrasonic instrument.”In a flaw-detection instrument the display may be an oscilloscope with limitedadaptability. The display, shown in Figure 16.10(a) resulted from a well-dampedtransducer. If the transducers incorporate less damping or if they are electronicallytuned, the echoes will contain more RF (radio frequency) cycles. In generalterms, the result is rectified and detected prior to the display stage, as shown inFigure 16.10(b). This results in the A-scan, which is simply amplitude versus timefor the echoes falling in the range of the transducer.Analog processes are incorporated in commercial flaw-detection instruments toanalyze data. In addition to the rectification of detection of the echoes describedabove, another process is that of a baseline suppressor that eliminates a few decibelsof the amplitude just above zero amplitude. This process serves to emphasizethe more significant flaw echoes, which are now easier to ascertain. A third processconstitutes the range gate that selects a time range from t 1 to t 2 along the A-scanbaseline. The signals within the range gate may be either (a) suppressed or (b) analyzedto the exclusion of all other signals. A computer, an internal alarm trigger, oran external analog instrument may execute the analysis. An alarm may be activatedif echoes fall within the range gate exceeds a certain amplitude or threshold.The A-scan procedure is also used to measure gauge thickness on the basis ofthe known velocity of the pulse inside the workpiece and the time it takes for anecho to be received from the backside of the workpiece.