## 3 Structural Monitoring via Global Navigation Satellite System: The Case of High Wind Turbines
### The Role of Frequency in Measurement  
In some structural control applications, the MR dampers are calibrated only once in the triggering phase of an extraordinary load (e.g., earthquake [59]), and then they behave passively during the remaining part of the dynamic excitation. In these cases, the role of the structural monitoring is of secondary importance, if not null. Nevertheless, the effectiveness of such systems generally is reduced compared to those cases where the continuous optimal calibration of the devices is done, adapting them to the instantaneous stress state. In the latter case (i.e., that of this research), the frequency of calibration over time can play a decisive role in the effectiveness of the structural control, and depends on the frequency of measurement of the dynamic response parameters involved in the control algorithm. In order to evaluate this incidence with reference to the case study, nine cases of SA control have been considered, each assuming a different frequency of acquisition (from 10 to 1000 Hz) of the structural response data needed to make the controller decide the optimal calibration of MR dampers. A value of latency equal to zero has been assumed, as well as 1 mm for measurement accuracy (best case). In short, in each test, measurements are made with different monitoring systems, and a different sampling rate, fixed around the typical values for commercial devices (Figure 3). The 1000 Hz case has been considered as an ideal case, being hard to implement in real applications. Figure 5: Equivalent base acceleration time history for the extreme operating gust (EOG) wind load case. Table 3 compares the structural responses of the wind turbine under the same EOG wind load, in both the fixed base and SA control configurations. When the acquisition is slower, the efficacy of the SA control strategy is lower in terms of reduction in base stress demand (reduction of 84% for command at 1000 Hz, of 50% at 10 Hz); even if in the worst case (10 Hz), the peak value is still half than that recorded in the fixed base configuration. This is explained by the fact that the controller can calibrate the MR devices much less frequently when low sampling rate devices are used for measurements, as confirmed by the number of switches ON\(\rightarrow\)OFF and OFF\(\rightarrow\)ON in Table 3. This means that the configuration of MR dampers during the motion is far from optimal most of the time (note the number of operations commanded by the control logic: 62 at 10 Hz, 1648 at 1000 Hz). From the last column of the table, one can observe that the base rotation is not significantly different in all the cases. This is an important result: even in cases where the measuring system is not performing and does not allow frequent control of the system, the latter's behaviour has no dangerous deviations for the safety of the devices placed at the base of the tower, i.e., for the overall system. The top displacement demand results are not so dependent on the sampling time of the monitoring system. Figure 6 shows the trend of the response parameters over time for the most representative of the above cases (a selection has been made to ensure the readability of the figures). It is worth highlighting that the base stress demand over time is much more variable in the case of a lower sampling rate of measurements. This fact may determine a higher risk of failure due to cyclic damaging and fatigue. The effect of the sampling rate on the displacement demand is less evident. To understand how much the load case intensity can count in the current investigation, the sensitivity analyses for variable frequency were repeated for a wind action equal to 75% and then 50% of the reference's extreme load case. Figure 7 shows the results obtained for each of the nine acquisition frequencies. The horizontal lines in parts (a) and (b) of the figure show the peak values of the structural response of the fixed base configuration for each of the three load cases.