Signal Diversity for Laser-Doppler Vibrometers with Raw-Signal Combination

Abstract

The intensity of the reflected measuring beam is greatly reduced for laser-Doppler vibrometer (LDV) measurements on rough surfaces since a considerable part of the light is scattered and cannot reach the photodetector (laser speckle effect). The low intensity of the reflected laser beam leads to a so-called signal dropout, which manifests as noise peaks in the demodulated velocity signal. In such cases, no light reaches the detector at a specific time and, therefore, no signal can be detected. Consequently, the overall quality of the signal decreases significantly. In the literature, first attempts and a practical implementation to reduce this effect by signal diversity can be found. In this article, a practical implementation with four measuring heads of a Multipoint Vibrometer (MPV) and an evaluation and optimization of an algorithm from the literature is presented. The limitations of the algorithm, which combines velocity signals, are shown by evaluating our measurements. We present a modified algorithm, which generates a combined detector signal from the raw signals of the individual channels, reducing the mean noise level in our measurement by more than 10 dB. By comparing the results of our new algorithm with the algorithms of the state-of-the-art, we can show an improvement of the noise reduction with our approach.

Modeling De-Coring Tools with Coupled Multibody Simulation and Finite Element Analysis

“…LDV sensitivity is set at 10 mm/s/V for measuring the hammer arm vibration and at 1000 mm/s/V to read the chisel vibration, as plotted in Figure 10. The quality of the measured signal is degraded due to the speckle effect, a common measurement error using an LDV, which causes signal dropouts [23]. The loss of the reflected waves causes sharp peaks in the measurement, seen in the chisel measurement as single sharp peaks.…”

Suppression of speckle noise in laser Doppler vibrometry by signal diversity and dynamic ellipse fitting

“…Thus, speckle noise restricts the accuracy and the resolution of LDV. Signal diversity and fusion of demodulation data from different channels are considered to be effective methods to suppress speckle noise because multiple independent reception channels reduce the probability of speckle noise [11][12][13].…”

Statistical behaviour of laser Doppler vibrometer detector signals and application of statistics for signal diversity

“…After combining the signals from independent channels, the errors can be drastically reduced. In out-of-plane LDVs, signal diversity was introduced to reduce signal dropouts and speckle noise [17,18,19]. We also extended the diversity method to an in-plane LDV in our previous publication [20].…”