Publications by authors named "Christian Rembe"

6 Publications

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

Sensors (Basel) 2021 Feb 2;21(3). Epub 2021 Feb 2.

Institute of Electrical Information Technology, Clausthal University of Technology, Leibnizstr. 28, D-38678 Clausthal-Zellerfeld, Germany.

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.
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http://dx.doi.org/10.3390/s21030998DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7867302PMC
February 2021

A Contactless Laser Doppler Strain Sensor for Fatigue Testing with Resonance-Testing Machine.

Sensors (Basel) 2021 Jan 5;21(1). Epub 2021 Jan 5.

Institute of Electrical Information Technology, Clausthal University of Technology, Leibnizstr. 28, D-38678 Clausthal-Zellerfeld, Germany.

In this article, a non-contact laser Doppler strain sensor designed for fatigue testing with the resonance-testing machine is presented. The compact sensor measures in-plane displacements simultaneously from two adjacent points using the principle of in-plane, laser-Doppler vibrometry. The strain is computed from the relative displacements divided by the distance between these two points. The optical design, the mathematical model for estimating noise-limited resolution, the simulation results of this model, and the first measurement results are presented. The comparison of the measurement results of our sensor with the results of a conventional strain gauge shows that our design meets the measurement requirements. The maximum strain deviation compared to conventional strain gauges of the laser-Doppler extensometer is below 4×10-5 in all performed experiments.
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http://dx.doi.org/10.3390/s21010319DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7796465PMC
January 2021

Compressional-Wave Effects in the Operation of a Quartz Crystal Microbalance in Liquids:Dependence on Overtone Order.

Sensors (Basel) 2020 Apr 29;20(9). Epub 2020 Apr 29.

Institute of Physical Chemistry, Clausthal University of Technology, Arnold-Sommerfeld-Str. 4, D-38678 Clausthal-Zellerfeld, Germany, (F.M.).

The operation of the quartz crystal microbalance (QCM) in liquids is plagued by small flexural admixtures to the thickness-shear deformation. The resonator surface moves not only in the transverse direction, but also along the surface normal, thereby emitting compressional waves into the liquid. Using a simple analytical model and laser Doppler vibrometry, we show that the flexural admixtures are stronger on the fundamental mode than on the overtones. The normal amplitude of motion amounts to about 1% of the transverse motion on the fundamental mode. This ratio drops by a factor of two on the overtones. A similar dependence on overtone order is observed in experiments, where the resonator is immersed in a liquid and faces an opposite planar wall, the distance of which varies. Standing compressional waves occur at certain distances. The amplitudes of these are smaller on the overtones than on the fundamental mode. The findings can be rationalized with the tensor form of the small-load approximation.
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http://dx.doi.org/10.3390/s20092535DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7249204PMC
April 2020

Non-Contact Damage Detection under Operational Conditions with Multipoint Laservibrometry.

Sensors (Basel) 2020 Jan 28;20(3). Epub 2020 Jan 28.

Institute of Electrical Information Technology, Clausthal University of Technology, 38678 Clausthal-Zellerfeld, Germany.

Scanning laser-Doppler vibrometry (SLDV) can localize and visualize damages in mechanical structures. In order to enable scanning, it is necessary to repeat the vibration. Therefore, this technique is not suited to detect emerging hazards in working machinery that change the vibration behavior. A common technique for such cases is monitoring the vibration excited by machine operation with accelerometers. This technique requires mechanical coupling between sensors and the measurement object, which influences the high-frequency vibration responses. However, in the low-frequency range, local damages do not shift resonances or distort operational deflection shapes (ODS) significantly. These alterations in the vibration behavior are tiny and hard to detect. This paper shows that multipoint laservibrometry (MPV) with laser excitation can measure these effects efficiently, and it further demonstrates that damages influence ODSs at frequencies above 20 kHz much stronger than at frequencies below 20 kHz. In addition, ODS-based damage indices are discussed; these are highly sensitive to minute visible changes of the ODSs. In order to enhance the sensitivity of hazard detection, the response vector assurance criterion value is computed and evaluated during operation. The capabilities and limitations of the methodology on the example of a cantilever with manually emerging damage are demonstrated.
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http://dx.doi.org/10.3390/s20030732DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7038392PMC
January 2020

Superresolution reflection microscopy via absorbance modulation: a theoretical study.

Opt Express 2018 Mar;26(5):5327-5341

Absorbance modulation enables lateral superresolution in optical lithography and transmission microscopy by generating a dynamic aperture within a photochromic absorbance-modulation layer (AML) coated on a substrate or a specimen. The applicability of this concept to reflection microscopy has not been addressed so far, although reflection imaging exhibits the important ability to image a wide range of samples, transparent or opaque, dielectric or metallic. In this paper, a simulation model for absorbance-modulation imaging (AMI) in confocal reflection microscopy is presented and it is shown that imaging well beyond the diffraction limit is feasible. In addition, we derive analytical design equations and estimate the dependence of the achievable resolution and pixel dwell time on relevant parameters, such as the AML properties and the applied light powers. We prove the validity of these equations through a comparison with the simulation results and we show that a resolution enhancement down to 1/5 of the diffraction limit is possible.
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http://dx.doi.org/10.1364/OE.26.005327DOI Listing
March 2018

Laser Doppler vibrometry as a noncontact method to detect various degrees of atrioventricular block: a feasibility study.

Future Cardiol 2016 05 22;12(3):269-79. Epub 2016 Apr 22.

Dipartimento di Ingegneria Industriale e Scienze Matematiche, Università Politecnica delle Marche, Piazza Roma, 22, Ancona, Italy.

Background: The vibrocardiography (VBCG) is a laser-based technique to monitor the heart rhythm without any contact to the body. The aim of this study was to evaluate whether the VBCG is able to detect the vibration patterns of the atria.

Methods: Simultaneous recordings of the ECG and VBCG in two cohorts were evaluated.

Results: The VBCG delivered a robust vibration pattern of the atrial contraction. A reliable determination of the interval and the different stages of an atrioventricular block was possible.

Conclusion: This is the first study that demonstrates the feasibility of a noncontact registration of the atrial vibration pattern. It enables a reliable determination of the atrioventricular interval. The VBCG can, therefore, serve as full noncontact monitoring.
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http://dx.doi.org/10.2217/fca-2015-0008DOI Listing
May 2016