1,720,957 research outputs found
Uncertainty mitigation in drone-based 3D scanning of defects in concrete structures
No full textThe measurement of defects in a concrete structures is highly relevant to determine how maintenance interventions should be performed. However, it could be difficult and potentially dangerous to inspect a certain structure by bringing trained operators, to places that are difficult to access. This issue could be overcome by framing the parts of interest of a building with a drone equipped with cameras. Nonetheless, a quantitative measure of a defect cannot be obtained with 2D cameras, since the pixel to millimeters scale and the estimation of depth are missing. To obtain a 3D shape measurement of a defect, 3D scanners, joined with 3D reconstruction, could be applied. In this article, we present a metrological evaluation of low-cost Time-Of-Flight (ToF) sensors for defects in concrete structures measurement. The defects of interest for this class of 3D scanners are mainly related to concrete spalling. This type of scanners was assembled on a drone with an onboard acquisition system. The testing benchmark for this study is based on a real structure with concrete spalling defects. A ground truth 3D model was obtained with a high-precision 3D scanner, used with a scaffolding. The effect of disturbances on measures were investigated, as well. The results of drone tests show that the systematic error of the 3D reconstruction with the selected sensors is about 0.5-2 mm, with a dispersion of raw data around the 3D reconstruction of about 2-4.5 mm, at a distance from the target of about 1.8-2 m
Feasibility Study of Drone-Based 3-D Measurement of Defects in Concrete Structures
Full text linkRecognition of defects in concrete structures, identification of cracks, concrete spalling, or other geometrical defects are important tools for structural damage detection. Defects in structures can include cracks, but also missing parts, due to the wear caused by weather or aging phenomena. These last types of defects in structures can be identified using red-green-blue (RGB) cameras, but the level of damage could be difficult to evaluate with 2-D images. In this sense, the application of 3-D reconstruction techniques can be helpful to determine the 3-D dimensions of spalling, swelling of concrete or the presence of visible steel parts of reinforced concrete. The use of drones for this type of measurement is very attractive for reducing the costs and time of measurement campaigns. However, the lack of accurate trajectory information and the vibrations affect the accuracy of 3-D measurements. In this article, a metrological characterization of measurement systems for the evaluation and recognition of defects in concrete structures is presented, starting from the acquisition of 3-D point clouds by low-cost time-of-flight (ToF) sensors, placed on drones. To evaluate the uncertainty of these systems, a mock-up with realistic defects was developed and characterized using a reference 3-D scanner. The 3-D reconstructions obtained via the selected sensors were used to evaluate the discrepancies of the 3-D shape compared to a ground truth model and the uncertainty of the selected scanners. The results show that, for all the defects tested, the standard deviation of the discrepancies between the defect reconstructed using the drone and the ground truth is below 2.5 mm
Virtual simulation benchmark for the evaluation of simultaneous localization and mapping and 3D reconstruction algorithm uncertainty
Full text linkSimultaneous localization and mapping (SLAM) algorithms allow us to obtain a unique 3D shape and 3D sensor trajectory by combining partial scans obtained by moving a 3D scanner. The performances of these algorithms are significantly affected by experimental conditions, characteristics of the target and values of the parameters of the reconstruction algorithm. Therefore, the uncertainty and reliability of SLAM techniques need to be assessed before their application, e.g. for robot navigation, autonomous vehicles or industrial fields. To evaluate the uncertainty of these algorithms, specific datasets containing 3D scans, with the possibility to control different conditions, e.g. sensor trajectory, depth or color noise, sensor velocity and framerate, are necessary. In this article, we present a procedure to obtain virtual datasets with complete control of the environment, 3D sensor and trajectory conditions, starting from any real 3D dataset acquisition, characterized by a sufficiently low uncertainty. These datasets can be generated to test the effect of SLAM algorithm parameters to determine the best parameters to be used to exploit the algorithm characteristics to obtain the best result in each operating context. The advantage of this procedure is the possibility to perfectly control each condition and to evaluate its effect on the final result. This procedure was applied to two reconstruction algorithms as examples; namely, the Open3D reconstruction tool and ElasticFusion. The results demonstrate that the setting of algorithm parameters, e.g. the tolerance on depth correspondence between frames or the number of fragments, or the change in number of frames acquired, can have a strong influence on the resulting 3D reconstruction and trajectory. Moreover, the effect of not closing the loop trajectory on reconstruction performance is quantified for different application scenarios
Feasibility of Drone-Based Modal Analysis Using ToF-Grayscale and Tracking Cameras
Full text linkThe application of modal analysis is a well-established procedure for detecting damage in civil structures. However, implementing monitoring systems on civil structures could involve a large number of sensors leading to expensive and cumbersome setups. On the other hand, unmanned aerial vehicles (UAVs) are already employed for monitoring purposes. Moreover, using UAVs inspections can be executed close to structures and using a single measurement node that can be moved from one measurement point to another. In this article, a vision-based measurement procedure composed of a system mounted on a prototype drone is presented in order to perform modal analysis of a structure. This measuring system is equipped with a grayscale camera, a time-of-flight (ToF) sensor, and a trajectory tracking camera. This composition is necessary to measure vibrations by tracking features on the structure (grayscale camera) to convert vibrations from pixels to millimeters (ToF sensor) and to track the trajectory of the drone (trajectory tracking camera). This latter sensor is used to reduce the effect of the movement of the drone in relation to the vibrations of the structure and to reconstruct the 3-D geometry of the structure. The grayscale camera on the drone is synchronized with another fixed grayscale camera, that frames a point on the structure, used as the reference point for modal analysis. This measurement procedure was tested on a mock-up of a structure a few meters in size, and the results are compared with those obtained using accelerometers. The outcome of these tests proves that it is possible to correctly measure resonance frequencies (with errors lower than 0.3%) and to reconstruct mode shapes with cameras assembled on a drone, as well as to reconstruct the 3-D geometry of the points measured
Automatic measurement of hand dimensions using consumer 3D cameras
Full text linkThis article describes the metrological characterisation of two prototypes that use the point clouds acquired by consumer 3D cameras for the measurement of the human hand geometrical parameters. The initial part of the work is focused on the general description of algorithms that allow for the derivation of dimensional parameters of the hand. Algorithms were tested on data acquired using Microsoft Kinect v2 and Intel RealSense D400 series sensors. The accuracy of the proposed measurement methods has been evaluated in different tests aiming to identify bias errors deriving from point-cloud inaccuracy and at the identification of the effect of the hand pressure and the wrist flexion/extension. Results evidenced an accuracy better than 1 mm in the identification of the hand’s linear dimension and better than 20 cm3 for hand volume measurements. The relative uncertainty of linear dimensions, areas, and volumes was in the range of 1-10 %. Measurements performed with the Intel RealSense D400 were, on average, more repeatable than those performed with Microsoft Kinect. The uncertainty values limit the use of these devices to applications where the requested accuracy is larger than 5 % (volume measurements), 3 % (area measurements), and 1 mm (hands’ linear dimensions and thickness)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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