Breaking the Excitation Chirp to Improve Ultrasonic Testing

Signal-to-noise ratio and axial resolution both play a primary role in ultrasonic exploration: the former is directly related to the depth of signal penetration in the innermost part of the subject under test. The latter improves with an increase of the bandwidth of the signal: this implies, however, an increase in the amount of noise superimposed on the useful signal. A tradeoff choice must be made between axial resolution (signal bandwidth) and maximum penetration depth (signal-to-noise ratio). The use of coded signals and appropriate signal processing techniques, as the pulse compression, overcomes this conflicting choice. Such use, although beneficial for the above aspects, implies that signals of relatively high duration are placed at the input to the measurement system, and this leads to an additional possible problem, related to overheating of both the object being measured and the transducer itself. The present work addresses this aspect concerning the use of coded signals and related processing techniques. By exploiting the linearity of the pulse compression procedure, a novel technique is proposed to deal with the problem of probe temperature rise in the case where very high penetration capacity and resolution are to be obtained while respecting the constraints imposed on temperature rise. The technique consists in fragmenting the Chirp signal into temporal parts, and then recomposing the signal through an appropriate processing technique. The simulation results reported show the validity of the proposed approach.