Quantifying the Performance of State of the Art Sensors to Measure Guided Elastic Waves

Elastic guided waves are carriers of information of the (change in) condition of plate-like structures like wind-turbine blades, airplane wings and road surfaces on bridges. To measure these guided waves we do not have to use piezos. Other sensors offer interesting benefits like contactless sensing, embedding, or measuring without electricity. However, quantitively comparing them is not trivial: the sensors all have different geometries, operating principles and are sensitive to different mode shapes of guided waves. We designed and performed an experiment to quantitatively compare the performance of five state of the art sensors (piezo, in-fiber interferometer, FBG, free-space interferometer, and ring resonator sensors) to measure So and Ao guided elastic waves. The measurements were performed on guided waves in an 8 mm steel plate, in the 60-150 kHz range. The dimensions of the plate and the positioning of the sources and sensors was chosen such that the So and Ao waves arrived in separate time windows. The in-fiber interferometer was the sensor that came closest to the piezo, that was used as reference sensor (-11 dB difference in SNR), the other optical based sensors have SNR values below -30 dB compared to the piezo. The measurements and simulations show that it is important to have two quantitative SNR measures for the performance to measure guided waves: one for the So and one for the Ao wave. For one sensor we found a difference of 22 dB between these two SNR measures.