Large engineering structures are often exposed to extreme wind loads, which can cause early damage and pose risks to the national economy and public safety. Laser Doppler vibrometry (LDV) has become a popular method for conducting ambient vibration testing (AVT) on large structures, such as historical buildings, due to its high sensitivity and accessibility. LDV allows non-contact vibration measurements acquired on remote surfaces by measuring the Doppler frequency shift of a reflected laser beam. However, LDV's performance for large structures is limited when it comes to partially inaccessible surfaces, where the LDV signal is weak due to low incident laser beam angles when the LDV is mounted on the ground. Using a drone to carry the LDV has shown promise to address this limitation. However, the most compact industrial LDV available on the market has a range of only 20-30 meters, and it requires a relatively heavy drone (6-10 kg of net payload) with stronger propeller motors, leading to significant background noise for the LDV and insufficient sensitivity for AVTs.

We are developing a new LDV-based measurement system called Flyable Mirrors to address these challenges. Flyable Mirrors project aims to improve AVTs of large structures by utilizing a commercial LDV mounted on the ground and an optical beam steering unit carried by a drone (Figure 1). The optical head of the drone includes motorized mirrors for scanning inaccessible surfaces with unrestricted incident angles. The reflective mirror is lightweight, requiring microdrones with payload capacities of only a few hundred grams. Additionally, microdrones have significantly lower vibration noise compared to heavy drones with several kilograms of net payload. We have conducted successful experiments such as in Figure 2. Vibration excitation from an eccentric shaker was remotely measured by Flyable Mirrors (based on LDV model Qtec® from Polytec® and DJI drone model M300) and a conventional accelerometer attached directly to the shaker table. There is a good match, about 90%, between Flyable Mirrors and accelerometer measurements for the low-frequency range up to 140 Hz, which is dominant for large structures.

Figure 3 shows an additional application for Flyable Mirrors. The system provides a rapid vibrational activity map for collapsed structures to detect human sounds and activities beneath them. This is particularly useful for rescue teams facing challenges accessing collapsed buildings due to their irregular shapes and large sizes.