Differential laser Doppler vibrometry with integrated alignment camera to reduce disturbances in airborne vibration measurements with a UAV

Vibration measurement on large and difficult-to-access structures is often associated with enormous costs. Recently, the application of an airborne laser Doppler vibrometer (LDV), which consists of a ground-mounted LDV and a mirror attached to a drone, to measure the vibrations of such structures was presented. However, this experiment also showed that the measurement result is strongly influenced by the vibration of the drone and air density fluctuations. These disturbances can be principally reduced by using a differential laser Doppler vibrometer (DLDV), in which both beams are guided out of the LDV towards the unmanned aerial vehicle (UAV). DLDV reduces disturbances caused by vibrations of the UAV or by fluctuations in air density with new alignment means. In this article, we will present a DLDV that enables alignment and positioning the laser beam at the point of interest. This improvement allows to visualize the target point in the center of a camera image where also an invisible infrared laser beam impinges the specimen.