# Ultrasound-Based Smart Corrosion Monitoring System for Offshore Wind Turbines
## 4 Smart Ultrasound Sensor Design
### Probe Selection  
The selection of ultrasound probe or transducer needs to be done by considering the sensor characteristics, such as sensor dimensions, weight, dead zone duration, waveform duration, and peak frequency. The transducer operating frequency spectrum and the waveform duration are often provided by the manufacturer. The peak frequency of the transducer controls the penetrating power that affects the possible inspection thickness of test material, e.g., high-frequency ultrasound has low penetration power and might almost attenuate within the material. As at a fixed velocity in a perfectly elastic material the wavelength and frequency are inversely proportional, a change in the probe frequency will affect the sensitivity and the resolution to detect flaws in the material. Due to the diffraction of ultrasonic, the sensitivity is about half of the corresponding wavelength [38]. Thus, sensitivity and resolution are increased when probe frequency increases. The dead zone duration is the interval following the initial pulse where the transducer ringingprevents detection or interpretation of reflected echoes. This is a constraint with thinner size samples as it is better that the time interval between back-to-back echoes has to be out of the dead zone.  
As we are going to measure 5 mm thick samples, it is important to check the dead-zone of the selected probe. A 5 mm thick steel sample will introduce a delay between back-to-back echoes of \(2\times 5/5.9\times 10^{6}=1.69\,\mathrm{\SIUnitSymbolMicro s}\) (the speed of sound in steel is approximately \(5.9\times 10^{6}\) mm/s). This is the expected value of the ToF before corrosion takes place. This value is important when considering the dead zone of an ultrasonic probe. Dead zone value is highly dependent on the test conditions and instrumentation used. It is not normally given in the technical documentation of the probe and needs to be determined experimentally. The values shown on Table 1 correspond to the time when the ultrasound signal has almost completely settled (see Figure 2). However, it must be noted that the dead zone does not entirely invalidate the ultrasound data received on that interval. By proper filtering, and with the appropriate receiver's gain, it would be possible to extract echoes within the dead zone of the piezo.  
\begin{table}
\begin{tabular}{l c c c} \hline \hline
**Part Name** & **YS15-10** & **TSB10-06** & **V111** \\ \hline Manufacturer & YUSHI & OKOndt & Olympus \\ \hline Diameter (mm) & 10 & 6 & 13 \\ \hline Peak Frequency (MHz) & 7.86 & 10.2 & 8.44 \\ \hline \(-6\) dB Bandwidth (MHz) & 3 & 3 & 6.93 \\ \hline Waveform Duration \(-20\) dB Level (\(\mathrm{\SIUnitSymbolMicro s}\)) & 0.7 & 0.840 & 0.272 \\ \hline Dead Zone (\(\mathrm{\SIUnitSymbolMicro s}\)) & 2.6 & 2.4 & 3.5 \\ \hline Connector/Position & BNC/Top & Microdot/Side & Microdot/Side \\ \hline Weight (g) & 50 & 7 & 20 \\ \hline Price (€) & \(\sim\)100 & \(\sim\)100 & \(\sim\)600 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Performance and features of ultrasound probes.  
Figure 2: Dead Zone of an ultrasound probe.  
A comparative analysis of the performance specifications of three sensors has been done to select the most suitable one for the UT system as given in Table 2. As a result, V111 from Olympus [39] has been selected as the most suitable ultrasound probe for our system, specifically prioritizing its lower waveform duration (higher bandwidth) and sound pressure power. Having a higher dead zone compared to the other two sensors will not be a constraint in the real scenario as the system is designed for testing thickness around 40 mm.