Noncontact Dynamic Displacement Measurement of Structures Using a Moving Laser Doppler Vibrometer

Monitoring of bridge displacements is critical for safe and cost-effective railroad operations. One of the essential parameters for determining the serviceability of the bridges is the dynamic displacement of the bridge during train-crossing events. The traditional methods for bridge displacement measurement often utilize linear variable differential transducers (LVDTs). However, irregular terrain, remote and inaccessible locations, and the height of the railroad bridges make implementation of these sensors for displacement measurements inadequate, risky, and time-consuming, and sometimes not possible altogether. In recent years, the use of laser Doppler vibrometers (LDVs) in the field of bridge displacement measurement has drawn attention as an alternative. In these applications, the vibrometer is generally placed on a fixed-point reference close to the bridge. However, it is not always possible to locate a fixed reference perpendicular to the bridge span to measure transverse displacements, especially when the bridge spans over a large opening. Furthermore, LDV sensors require calibration for every unique, different setup and are cumbersome to implement across a variety of different bridges. This paper presents a novel concept for bridge displacement measurement that enables the use of noncontact and reference-free moving vibrometers in the field without the need for calibration. The concept discussed herein proposes a method of compensating for measurement errors due to the angular and linear movement of the vibrometer to obtain accurate transverse displacement measurements of bridges. The results of this study showed that the signal difference between the measured outputs of a moving LDV system and an LVDT was between 10% and 15% peak-to-peak and between 2% and 5% root-mean square (RMS), which are generally considered acceptable levels of accuracy by railroad managers for field applications. An outdoor test was conducted in which an LDV mounted on an unmanned aerial system (UAS) was used to collect displacement measurements of a moving target structure representing a railroad bridge train-crossing event. Researchers conducted three outdoor experiments by collecting LDV and LVDT measurements from the UAS as it was hovering. The results from this outdoor testing showed that the signal difference between the measured dynamic data from the moving LDV and the LVDT was less than 5% (peak) and 10% (RMS). The reference-free dynamic displacement is generally accepted by railroad managers as an important index related to bridge deterioration, so this application can inform railroads in the field of prioritization decisions without the need for installed sensors. The ultimate goal of this research was to validate the applicability of the proposed method by mounting one LDV on a UAS and measuring dynamic bridge displacements with minimal setup and calibration.