## 3 Application of Drive-by Methodologies on the Bridge Subsystem
### Bridge Damage Identification
#### 3.2.2 Feature Extraction
In their paper, [PERSON] et al. [22] analyzed the performance of two time-frequency algorithms in identifying damages in a short-span Warren truss bridge by using a FE 3D model to simulate the dynamic interaction of the track-train-bridge system. Numerical investigation is conducted, and two types of damage are simulated, the first of which involves the connection between the lower chord and the side diagonal member, and the second of which involves the connection between the cross-girder and the lower chord. Then, two different time-frequency algorithms, continuous wavelet transform (CWT) and the Hilbert-Huang transform (HHT), are used to analyze the vibration response of the train bogie. The performance of the two algorithms is then compared in terms of their ability to identify the location and severity of the damage. The results show that both the CWT and HHT algorithms can identify the location of the damage in the bridge model, but the HHT algorithm is found to be more accurate in identifying the severity of the damage. Figure 13 shows the position-frequency map of the difference between the damaged and healthy CWT coefficients and the HHT values. The damaged area may be identified by the index maxima in both instances, even if the HHT instantaneous energy peak is near the right edge. The authors discussed the limitations of the study, including the use of a simplified bridge model and the assumption of linear behavior in the vibration response of the bridge. Additionally, the article suggests possible avenues for future research in this area, including the use of more complex bridge models and the development of more robust time-frequency algorithms. Figure 13: Damage detection results in either the absence (**a**,**b**) or in the presence (**c**,**d**) of track irregularities with the train traveling at 100 km/h. The module of the difference in wavelet coefficients (**a**) and Hilbert spectrum (**b**) for a considered total damage on the fastening between the diagonal member and the lateral chord. The module of the difference in wavelet coefficients (**c**) and Hilbert spectrum (**d**) for a considered total damage on the fastening between the side wall diagonal member and the lateral chord [22]. [PERSON] [73] presented a drive-by inspection approach using vehicle parameter optimization and vehicle displacement as a damage indicator. The displacement difference profile of vehicle is obtained by subtracting the vehicle's time histories between the baseline and the damaged bridge, where the baseline is derived from the first run over the bridge in good condition. Through several simulations, it was determined that the occurrence of damages may be effectively detected as the peak in the displacement difference profile, whilst the damage level was considered to be the rise in displacement difference. This work provided an approach of parameter optimization aimed at enhancing the vehicle displacement profile difference and minimizing the noises from vehicle dynamics to increase the accuracy and sensitivity of detection findings. The findings suggested that the optimal vehicle characteristics may significantly enhance the inspection performance for all damage situations. Using Monte Carlo techniques, the optimal parameters were established by running several simulations under various damage scenarios. Based on the vehicle displacement profile, the results demonstrated that bridge defects may be identified efficiently and precisely by applying the ideal vehicle characteristics. The work presented by [PERSON] et al. [74] investigated the potential of using repeated dynamic measurements taken on a passing train to determine the condition of a bridge. The paper presented a full-scale field study of drive-by bridge monitoring using acceleration measurements on an instrumented train. Ensemble responses of the traversing train signals were indicative of the existence and location of a stiffer part of the viaduct where two spans were replaced.