## 5 Conclusions  
This work focuses on alternative structural monitoring systems for the semi-active control of wind turbines. Indeed, a strategy for mitigating the structural demand to high turbines proposed in the past by the authors requires continuous monitoring of the demand for base stress and top displacement of the tower. The role that the monitoring system performance can play in limiting or favoring the performance of the \"smart\" structural protection technique has been investigated herein. To this end, a comparative analysis of the products and monitoring solutions suitable for the purpose (GNNS) and available on the market was first conducted, with particular reference to the characteristics of greatest interest for the specific application (frequency of acquisition, latency, accuracy of measurements). Based on this investigation, an analysis of the sensitivity of the effectiveness of the structural control system to changes in the properties of the monitoring system was then carried out. This was made with reference to a realistic case study of a wind tower about 100 m high, and was solicited by an extreme gust. The performance of the measurement system in terms of acquisition frequency, latency, and accuracy were assessed separately, one at a time. With regard to the last aspect, the numerical simulations have shown that the performance of the control system is not tangibly affected by the accuracy of the measurements, even when these are the standard for entry level devices (20 mm). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Load Case vs. Full EOG** & \(\sigma_{\text{max, SA}}\)/\(\sigma_{\text{max, FB}}\) & \(\chi_{\text{max, SA}}\)/\(\chi_{\text{max, FB}}\) \\ \hline
50\% & 24\%–42\% & 37\%–39\% \\
75\% & 21\%–68\% & 34\%–61\% \\
100\% & 25\%–85\% & 50\%–77\% \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Range of demand reduction against the three load cases for the range 10–100 ms of latency in measurements for control. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Maximum Base** & **Maximum Top** & **No. of State** & **Maximum Base** \\  & **Stress** & **Displacement** & **Changes** & **Rotation** \\  & **[MPa]** & **[mm]** & **(On/Off)** & **[rad]** \\ \hline Fixed base & 90 & 1320 & - & - \\ SA–no latency & 14 & 753 & 1648 & 0.0054 \\ SA–lat. 10 ms & 22 & 731 & 366 & 0.0053 \\ SA–lat. 20 ms & 36 & 656 & 220 & 0.0046 \\ SA–lat. 30 ms & 33 & 701 & 156 & 0.0051 \\ SA–lat. 50 ms & 39 & 685 & 104 & 0.0049 \\ SA–lat. 60 ms & 76 & 1019 & 114 & 0.0050 \\ SA–lat. 70 ms & 49 & 772 & 86 & 0.0056 \\ SA–lat. 80 ms & 38 & 700 & 70 & 0.0052 \\ SA–lat. 100 ms & 45 & 659 & 62 & 0.0052 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Peak response of the wind turbine by varying the latencies of measurements. The sampling rate and latency can play, however, a crucial role, since they may lead to an ineffective calibration of the variable devices, because they are not timely. It must be said that, even in the worst cases (10 Hz frequency and 100 s latency), the response of the \"controlled\" wind tower is no worse than it would have in the standard \"fixed base\" configuration. However, the advantages of adopting the proposed special structural protection system are reduced. It has been observed that when a high-performance monitoring system is adopted, the structural response in terms of stresses at the base and displacements at the top turns out to be consistent with the limit values that the control logic is calibrated and based on. As the performance of the measuring system worsens, the stresses also exceed the threshold values by two or three times, while remaining lower than those of the \"fixed base\" system. The response in terms of top movement is also affected, but to a lesser extent. By comparing the structural response against different wind load intensities, it has also been shown that the worsening of this response to the worsening of the monitoring system's performance is gradually more accentuated as the intensity of the external action increases.