Investigation of simultaneous signal transmission in non-destructive inspection of steel billet

A non-destructive method of steel billet, which uses time-of-flight of ultrasonic longitudinal wave and detects defects as decrement of pseudo sound velocity reconstructed by computerized tomography, has proposed. The remaining problem is long measurement time, owing to step-by-step measurement corresponds to each sound path whose number becomes huge. One of the solutions is simultaneous measurement, which can be achieved by transmitted signals simultaneously. To avoid interference among signals, choice of the transmitted signal becomes one of the most important points to be considered. In the case of non-destructive inspection of steel billet, many reflected waves from boundaries may cause adverse influence. Therefore, in this study, the parameters of the signal; modulation scheme, modulated signal, signal length, and signal frequency are investigated. Simulation-based study suggested that the signal is required to be short and independent each other. From those conditions, some experiments about defect detection by the several kinds of modulation schemes using pseudo-random noise were conducted. As a result, defects detection by 5ch simultaneous transmission was successful. It was suggested that the total measurement time could be decreased to 1/5.