## 3 Methodology
### Descriptions of the Tests
#### 3.2.1 Small-Scale Testing  
Two experiments were conducted in the structural laboratory at Universidad del Norte: The first experiment resulted in the analysis of vibration data recorded from a set of frequency chirps by a shaking table, whereas the second experiment considered the dynamic characterization of a flexible structure.  
* _Slaking Table Tests_: The Quanser Shake Table II is a single-axis motion simulator operated with open architecture software. The shaking table can reproduce sinusoidal and frequency chirp motions and pre-loaded accelerograms of real earthquakes employing a plan dimension platform of 61 \(\times\) 46 cm with a total height of 13 cm. The simulator allows an operational weight of 7.5 kg for an acceleration of 2.5 g [32]. This test ensued in the analysis of data vibrations recorded from frequency chirps performed by the shaking table with different displacements according to distinct frequencies from 0.5 to 5.0 Hz, 0.5 Hz increments, using a sinusoidal excitation. For the frequency chirps, 0.10 cm, 0.25 cm, 0.50 cm, and 0.75 cm displacements were considered. Two wireless measuring nodes and one piezoelectric accelerometer were used to deploy the low-cost and reference SHM systems during testing. The accelerometer arrangement over the Shake Table II platform is presented in Figure 4.  
\begin{table}
\begin{tabular}{l l l l} \hline \hline
**Feature** & & **Accelerometers** & \\ \hline Model & 356B18 & 356B18 & 333B50 \\ Measurement axes & Triaxial & Triaxial & Uniaxial \\ Reference name & PCB L94 & PCB L74 & PCB L12 \\ Sensitivity (mV/g) & 1000 & 1000 & 1000 \\ Measurement range (V) & \(\pm\)5.0 & \(\pm\)5.0 & \(\pm\)5.0 \\ Frequency range \(\pm\)5.0\% (Hz) & 0.5–3000 & 0.5–3000 & 0.5–3000 \\ Cost (USD) & \$1764 & \$1764 & \$598.5 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Technique features of the PCB Piezotronics accelerometers.  
Figure 4: Experimental setup of the frequency chirps on Quanser Shake Table II.  
The comparison of the acceleration time series between the PCB L74 accelerometer and the ADXL355 RBP1 and ADXL355 RBP4 accelerometers was conducted through three types of acceleration spectrum in the direction of motion: from the original sampling frequency, from 100 Hz resampling frequency, and from the mean of the highest 100 acceleration peaks from 100 Hz resampling frequency. Then, the RMS error of vibration data was calculated from wireless measuring nodes accelerometers regarding the piezoelectric accelerometer. Finally, spectrograms were computed for time-frequency distribution analysis.  
* _Flexible Structure Monitoring_: The tested two-story structure consists of 15 \(\times\) 10 cm acrylic plates as floor systems and 80-cm aluminum leaves as lateral support elements. The story height is 40 cm, and the structure was constrained at the bottom above a stiff desk by clamps. The flexible structure was represented as a two-degree-of-freedom (2-DOF) shear frame for the testing. The dynamic characterization of the structure was carried out through ambient and free-vibration records from the structural response, excited by random displacements on the highest DOF. The experimental setup involved a measurement channel on every DOF, and every measurement channel had a measuring unit for each SHM system. Figure 5 shows the aluminum structure, the experimental scheme of the testing, and the sensor array on DOF 2 of the flexible structure.  
The vibration data of the reference SHM system were resampled at a 250 Hz sampling rate to perform the dynamic characterization of signals of frequency content without bias. As a 2-DOF dynamical system response was measured, it was possible to identify dominant vibration modes corresponding to the directions of motion. Ambient vibration data were processed using the FDD technique to estimate natural frequencies and mode shapes. Likewise, free-vibration data were processed by the logarithmic decrement method to calculate modal damping ratios.