Project Title: Developing Airborne Laser Doppler Vibrometry for Enhanced Dynamic Characterization for Large Engineering Structures
DFG individual research project No. 684101/2022 with three years funding from July 2022 to June 2025.
Principle Investigator: Dr. -Ing. Mohamed Ismail
Motivation
Large engineering structures are subjected to extreme wind loads that may lead to early damages and hazards for the national economy and public safety. A class of these structures, e.g., large heritage structures, is characterized by a high uncertainty level for degradation performance due to limited structural-aging data. The level of knowledge can be increased by measuring the actual structural properties by ambient vibration testing (AVT). The structural dynamic response from AVT is then utilized for updating numerical degradation models that can better identify the ongoing aging mechanism and the remaining useful life prior to interventions or damages. Recently, laser Doppler vibrometry (LDV) has been widely utilized for conducting AVT for large structures, e.g., towers and large bridges, thanks to its superior sensitivity and accessibility compared to a network of tens-to-hundreds of mounted displacement sensors. The principle of the LDV is based on calculating the Doppler frequency shift of a reflected laser beam due to the motion of the target surface. Finally, LDV provides real-time velocity or displacement data that represent the dynamic response of said surface. Two challenges limit LDV performance significantly for AVTs of large structures, namely: a) partially inaccessible surfaces in which the LDV signal has a weak strength because of low incident laser beam angles by an LDV mounted on the ground and b) significant optical deficiencies in long-range measurements due to air turbulence and laser speckle effects.
Research Objectives
This project aimed to enhance the structural dynamic characterization of large structures by exploring a new measurement method: airborne LDV realized by a sophisticated LDV with novel noise-suppression means at the ground and an optical beam steering unit carried by an unmanned aerial vehicle like a drone. The optical head at the drone involves motorized mirrors for scanning inaccessible surfaces with unrestricted incident angles via the drone. In addition, the project addresses the measurement of drone movements and vibrations. Compensation for such influences is a major project goal. Toward building and testing the first airborne LDV, several preliminary experiments have been jointly conducted via multidisciplinary research at the DLR Institute of Flight Systems and at the Applied Metrological Department at Technical University of Clausthal. The airborne LDV will enable new insights for monitoring structural-aging mechanisms that are not yet possible due to aforementioned deficiencies in ground LDVs.
This project has a → parallel follow-up DFG project managed by Prof. Christian Rembe at the Applied Metrological Department at Technical University of Clausthal.