Remote mechanical vibration sensing: A comparison between CW-doppler radar and laser-doppler vibrometer measurements

Vibration-based structural health monitoring (SHM) systems mainly use accelerometers or strain gauges for measuring the dynamic response of the structure. However, these types of sensors require a direct contact which limits their applicability in many applications like for example the analysis of rotating machinery. One opportunity to measure the electrodynamic vibration from a distance of several meters is to use Laser-Doppler vibrometers (LDV). However, due to the high price and the complexity of such LDV-systems it is not feasible to apply them in permanent sensor installations. A possible solution is the usage of radar sensor devices that can be used for remote mechanical vibration sensing either in a continuous wave (CW) or frequency-modulated continuous wave (FMCW) mode. A cheap production of the radar devices - as demonstrated by the automotive sector - makes their permanent application also feasible in SHM-applications. The goal of this paper is to compare - for the first time - two CW-Doppler radar sensors at 24 GHz and 94.5 GHz with a Laser Doppler vibrometer. In this paper, we have measured the performance of both Doppler radars and LDV simultaneously, and the result is validated using a force sensor which is directly attached to the tested specimen. Experimental outputs are shown and it is verified that there is a good agreement in results between all the four sensor types. Also, various harmonics are observed which are probably due to the artifacts. This experiment shows the potential of replacement of LDV with the low-cost Doppler radar system in SHM-applications.