Transducer-to-Transducer Communication in Guided Wave Based Structural Health Monitoring

Systems for guided wave (GW) based structural health monitoring are limited in the frequency bandwidth of the excitation signal due to the underlying wave’s dispersion that causes a broadening of the waveform along the waveguide. Hence, bandlimited waveforms such as toneburst or chirp signals are widely employed. Recently, the authors have investigated the potential of phase-modulated signals, known from CDMA-communication systems, as a possible way to overcome such limitation. It was found that this class of excitation enables a parallel transmission and reception of all piezoelectric transducers which may lead to a significant reduction in the overall system complexity, because of channel switching is not required anymore. In particular, by means of matched filtering a pulse compression and signal demodulation can be achieved. In addition, the signal-to-noise ratio can be improved by considering phase modulation signals of longer code length. In this paper, we investigate additional properties of phase-modulated signals in terms of their capability to transmit digital information on the health status of the structure through the structure itself [1]. This may lead to autonomous GW-based SHM system where the sensor nodes do not communicate with each other through a wireless module, but where the information are being delivered through the waveguide. A proof-of-concept will be demonstrated here on an isotropic plate using both simulated signals and experimental measurements. The implementation of the concept requires to compensate the guided wave signals from dispersion. Here a suitable Warped Frequency transform [2] designed on the group velocity of the guided modes, is applied. Such transformation has two beneficial effects: 1) it compensates for the detrimental effect of dispersion, 2) it preserves the pseudo-orthogonality of the encoded pulses, because it is computed with a unitary operator.