Measuring with laser Doppler vibrometer on moving frame (LDVMF)

Abstract

Structural dynamic gives insight into structural properties such as mass, eigenfrequencies, eigenmodes, modal damping and strain distribution and can be utilized in structural health monitoring, dynamic sub-structuring, etc. In this context structural vibration is measured and used. The measurement is done by means of conventional sensors such as accelerometers or non destructively using Laser Doppler Vibrometer (LDV), for instance. The non-destructive, non-contact measurement techniques preserve the integrity of the structure and don't add mass and stiffness to the structure under test. When one deals with civil structures such as rail and road ways, pipelines and catenary the importance of these techniques becomes more evident as they allow standoff measurement on a moving frame. Nevertheless when LDV is employed due to the relative in-plane motion between the LDV and the target speckle noise is generated which degrades the signal quality and makes this application not very straightforward but challenging. One of the first Laser Doppler Vibrometer on moving frame is adopted to measure and monitor the ground vibration, aiming at detection of buried land mines. The major addressed difficulty in this application is the speckle noise present in the acquired signal. In general the signal quality and the Signal to Noise Ratio (SNR) are a function of the laser spot size and wave length, measurement distance, relative velocity and sampling frequency. A trade-off between these factors, which are not always intuitive would help to minimize the noise floor due to the speckle noise. In this paper a test rig is presented which allows to study the speckle noise at different measurement ranges, between 1.8 and 2.8 m, and different velocities, up to 150 km/h. The results might serve as a guideline to the design process of a LDVMF.

Railway sleeper vibration measurement by train‐borne laser Doppler vibrometer and its speed‐dependent characteristics

“…The speed of such in‐plane motion is reported to be the most influential factor in speckle noise behavior (Martarelli & Ewins, 2006). Simulations and experiments show that the amplitude of speckle noise increases with the increase in in‐plane speeds (Jin et al., 2022; Rahimi et al., 2014; Sracic & Allen, 2009). In most LDV applications, speckle noise is mitigated through discrete‐point measurements (Revel et al., 2011; Sels et al., 2019), repeated closed‐path scanning (Allen & Sracic, 2010; Stanbridge et al., 2004), or surface treatment (Xu et al., 2017).…”

Removing speckle noise from the signals of a laser Doppler vibrometer on moving platforms (LDVom) by ensemble empirical mode decomposition

“…However, reciprocating or cyclical scans and cyclical artificial excitation cannot be realized in largescale measurements. In contrast, an LDV on moving platforms (LDVom) [24] can one-way continuously scan the sample surface, especially those long structures like railway tracks. This technique is totally different from the previous ones (compared in table 1) and provides the possibility for large-scale measurements.…”

“…This technique is totally different from the previous ones (compared in table 1) and provides the possibility for large-scale measurements. However, improving signal quality becomes a primary task for the LDVom [24].…”