186,430 research outputs found
Mode matching of Continuous Scanning Laser Doppler Vibration data in the frequency domain
No full textApplications as structural diagnostics, condition monitoring and fatigue testing are requiring the development of vibration tests characterized by reduced testing time, fine spatial resolution and high Signal to Noise Ratio (SNR). In this context, Continuous Scanning Laser Doppler Vibrometry (CSLDV) can have a great impact as a substitute of classic Discrete Scanning Laser Doppler Vibrometry (SLDV). In fact, CSLDV makes it possible to measure the target structural vibration much faster and with finer spatial resolution than SLDV, as well keeping an acceptable level of SNR. CSLDV joins together the spatial and time information, because the vibration datum obtained from the laser, which continuously scans (over time and space) the structure under test, is modulated by the Operational Deflection Shape (ODS) excited during the experiment. This results in a spectrum characterized by sideband patterns uniquely associated to the ODS excited. However, the current drawback in fully exploiting CSLDV in everyday testing is related to the necessity of being managed by an expert operator who knows how to extract meaningful information from data measured. This paper proposes a procedure which aims to automatize the information extraction process from CSLDV signals, in order to ease the utilization of CSLDV in vibration laboratories. The idea starts from a simple observation: if the mode shapes of the structure under test are known a priori, e.g. from a numerical model, an analytical formulation or previous measurements, as is the case for fatigue tests, it is possible to settle a procedure that searches for similarities between those known mode shapes (the candidate mode shapes) and ODSs that actually modulate the signal measured. This procedure can therefore be considered a pattern matching technique that is able to identify the resonance frequency related to each ODS and the mode shapes that better match with the ODSs excited. A detailed description of the algorithm is given in this paper. Moreover, the procedure is analyzed in order to discuss its sensitivity to noise, overlapping of resonance frequencies (close modes situation) and ODS complexity. The application of the approach to experimental data is also discussed
A survey on 360-degree video:Coding, quality of experience and streaming
No full textThe commercialization of Virtual Reality (VR) headsets has made immersive and 360-degree video streaming the subject of intense interest in the industry and research communities. While the basic principles of video streaming are the same, immersive video presents a set of specific challenges that need to be addressed. In this survey, we present the latest developments in the relevant literature on four of the most important ones: (i) omnidirectional video coding and compression, (ii) subjective and objective Quality of Experience (QoE) and the factors that can affect it, (iii) saliency measurement and viewport prediction, and (iv) the adaptive streaming of immersive 360-degree videos. The final objective of the survey is to provide an overview of the research on all the elements of an immersive video streaming system, giving the reader an understanding of their interplay and performance.</p
Continuous scanning laser Doppler vibrometry and wavelet processing for diagnostics: A time domain approach
No full textContinuous Scanning Laser Doppler Vibrometry (CSLDV) is a well-known technique within the structural dynamic community. However, the whole potentials of CSLDV for diagnostic purposes have not been fully exploited yet. This paper presents a time domain approach for identifying damages in structures. The method, which is based on a wavelet processing of vibration data collected by CSLDV, does not need any a-priori knowledge of the vibration behavior of the undamaged sample. Applications on real test cases are presented and discussed in the paper, demonstrating the promising performance of the approach as a non-destructive testing technique
Peak Age of Information Distribution Bounds for Multi-Connectivity Transmissions
No full textTransmission of packets over multiple wireless interfaces is an effective method to improve reliability and reduce the delay of Internet of Things (IoT) transmissions. In this context, Age of Information (AoI) has become a useful metric for many applications, measuring the freshness of the information available on a process that is measured by a remote sensor. In this work, we study the Peak Age of Information (PAoI) of an M/M/2 fork-join system, in which a sensor sends packets simultaneously over 2 separate queuing systems. The first packet to reach the receiver is considered as delivered. We derive lower and upper bounds on the PAoI for systems with finite and infinite buffers based on a low-traffic approximation, and show that the bounds are very tight at the optimal working point, so that the best rate derived from the bounds is very close to the optimum.</p
Mode filtering of Continuous Scanning Laser Doppler Vibration Data
No full textThe paper illustrates the idea of using theoretical knowledge of specific structure's mode shapes as a way of filtering time domain data obtained by Continuous Scanning Laser Doppler Vibrometry (CSLDV). The CSLDV output measures the structure vibration joining together the spatial and time information. It is proposed here to exploit the a priori knowledge of the candidate mode shape spatial distributions to extract from the vibration data the resonance frequency information with high accuracy. That technique is based on the concept that modal analysis ends up with a final abstraction and labelling of the mode shapes on the basis of their nodal lines position, i.e. first, second bending and/or torsional, etc. The expected mode is compared with the experimental data in order to evidence the information on the frequency at which that mode occurs
Exploiting Continuous Scanning Laser Doppler Vibrometry in timing belt dynamic characterisation
No full textAcoustic beamforming for noise source localization – Reviews, methodology and applications
No full textThis paper is a review on acoustic beamforming for noise source localization and its applications. The main concepts of beamforming, starting from the very basics and progressing on to more advanced concepts and techniques, are presented, in order to give the reader the possibility to identify concepts and references which might be useful for her/his work. Practical examples referring to application of this technique in different scenarios are also provided. The aim is to make the reader comfortable with the topic and aware of the wide stimuli a technique like acoustic beamforming can offer researchers
Underwater image pre-processing for photogrammetric application
No full textThis work describes a new method for correcting underwater images acquired under natural illumination condition and targeted to photogrammetry for 3D object reconstruction. In particular, a comparison between the new approach and a previous one from literature is presented. The new approach demonstrated to be a feasible and robust solution to improve quality, colour and contrast in underwater imaging by means of a three-step procedure. The focus of the paper is on 3D reconstruction of small-sized corals in shallow water. The colour of pre-processed images, with a visible green-blue colour cast, are enhanced and the higher images quality eases the whole object reconstruction acquired in a poor visible scene. The higher performance of the proposed method is experimentally demonstrated
Tuning of the functional beamforming resolution for wind tunnel measurements
No full textConventional Frequency Domain Beamforming (CB) is characterised by the frequency
dependency of the mainlobe width and by the presence of sidelobes that limit its dynamic
range. Functional Beamforming (FB) has been introduced with the aim to overcome these
limitations, narrowing the mainlobe and reducing the sidelobe levels. This paper introduces
a strategy to obtain a beamformer with a target mainlobe width that is constant over a
desired frequency range. The idea is to properly adjust the Functional Beamforming order
ν, frequency by frequency, to preserve the mainlobe width. A tuning procedure of the
order ν is presented and applied to a typical wind tunnel setup. A detailed analysis of the
dependency “order ν versus frequency” is discussed and a general formula describing this
dependency is provided. Finally, the effectiveness of the proposed approach is shown both
on simulated and experimental test case
Preliminary assessment of Photogrammetric Approach for detailed dimensional and colorimetric reconstruction of Corals in underwater environment
No full textThe use of 3D scan technologies to provide 3D color models of some bio-constructor organisms without removing them from the sea represents a valuable contribution for the preservation of current marine flora and fauna. The necessity to perform accurate measurements of volume, surface area and other morphometric characteristics is to increase the knowledge about the habitat-forming species. In this way, it is possible to describe the modification they made to the seabed topography, to monitor their shape modification according to local environmental conditions and evaluate 3D structure and color of some species sensitive to climate changes. The necessity of working in an environment characterized by different light conditions and particulate concentrations, the need of having high-performance sealing and, last but not least, the portability guarantee have limited the range of possible measurement techniques among those available for 3D shape reconstruction. Within this context, the idea underlying this research activity is to adopt photogrammetric technique for underwater surveys. In particular, a comprehensive and detailed analysis of a small object in and outside-water is performed in lab-scale tests by means of two different camera lenses, to pinpoint the effectiveness and the weakness of the solution identified. Another contribution of this paper is the uses of a color correction algorithm in order to correct the scene illumination, improving the 3D reconstruction of the detected objects. It is shown the capability of the algorithm to solve the color mismatch problem in multi-view color images. Results show the 3D models with details very well reconstructed both in and outside-water. The image pre-processing step drastically improve the matching points recognition and the reconstruction of the whole object independently to environmental illumination changes
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