# Drone-Based Vibration Monitoring and Assessment of Structures
## 4 Experimental Validation and Results
### Damage Detection Validation  
This experiment was designed to validate the ability of the sensor package to collect meaningful data for SHM applications. Since the system was developed to detect Level 1 damage, the sensor package needed to collect data before and after the structure had sustained damage. To simulate damage, three bolts supporting the steel plate were removed, as shown in Figure 7. This experiment also utilized the electromagnetic shaker to generate harmonic excitation forces. Additionally, a load cell (Ling Dynamic Systems V408) was placed between the shaker and the plate to record the force transmitted to the structure to provide ground truth measurements. The sensor package was then manually mounted directly underneath the electromagnetic shaker and load cell, in the position a0 shown in Figure 7. The shaker was first actuated at 7 Hz for 4 s and then stopped to allow the structure to vibrate freely. This was repeated at 14 Hz for both the undamaged and damaged structure.  
The collected data underwent preprocessing in which the data from each experiment were smoothed through a 50-point rolling average and resampled to a total of 200 points to be considered as the response, \(R_{N}\), in Equation (3). The associated forcing function, \(f_{N}\), in Equation (3) can be either the load cell record or the voltage signal that drives the linear actuator [31]. In this paper, the load cell record was utilized. Equation (3) can now be implemented on each record to extract the DSR of the structure. It has been shown that damage can be detected by processing even a short segment of the record [31], provided that the time step of the acquisition is adequate to capture the maximum desired frequency affecting the response.  
Figure 10 shows a 400-step segment of the DSR of the undamaged structure. Each DSR was extracted from the acceleration time history sampled in each of the three tests with the 14 Hz excitation force. As expected, the three DSR signals were almost identical and showed a high correlation (\(R^{2}\) = 0.989). This indicated that the state of the structure had not changed between acquisitions, and therefore, no damage was present. Figure 11 compares the DSR of the damaged and undamaged structure to the 14 Hz excitation force. These two BIRFs showed no correlation, indicating a change in the structural condition; thus, the presence of damage is detected.  
In Figures 10 and 11, the damage detection algorithm operated on a 0.8 s window of the free vibration structural response with an acquisition step of 2 \(\times\) 10\({}^{-3}\) s. This allowed for approximately 10 sampling points within a response cycle, and thus, this was the minimum acquisition step required to capture frequencies up to approximately 300 Hz. It is noted that the structural response record was resampled to 1/4 th of the original sampling frequency. It is also noted that an even shorter segment of the structural response could potentially be used in the damage detection algorithm.  
Figure 9: Frequency response function of the sensor package for 1–20 Hz excitation.  
Table 3 summarizes the \(R^{2}\) correlation coefficients of the BIRFs extracted from all tests for the two excitation frequencies on the undamaged and damaged structure. It was observed that all BIRFs correlated well (\(R^{2}>0.8\)) when extracted from responses of either the undamaged or damaged structure, for both frequencies. However, the correlation was completely lost (\(R^{2}\approx 0\)) when the BIRFs of the undamaged and damaged structures were compared, regardless of the excitation.  
Figure 11: The DSR extracted from 400 steps of the free vibration phase of the response of the undamaged and damaged structure excited by a 14 Hz harmonic force.  
Figure 10: The Dynamic Signature Response (DSR) extracted from 400 steps of the free vibration phase of the response of the undamaged structure excited by a 14 Hz harmonic force.