Time-optimal path parameterization (TOPP) is a well-studied problem in robotics and has a wide range of applications. There are two main families of methods to address TOPP: numerical integration ...(NI) and convex optimization (CO). The NI-based methods are fast but difficult to implement and suffer from robustness issues, while CO-based approaches are more robust but, at the same time, significantly slower. Here, we propose a new approach to TOPP based on reachability analysis. The key insight is to recursively compute reachable and controllable sets at discretized positions on the path by solving small linear programs. The resulting algorithm is faster than NI-based methods and as robust as CO-based ones (100% success rate), as confirmed by extensive numerical evaluations. Moreover, the proposed approach offers unique additional benefits: admissible velocity propagation and robustness to parametric uncertainty can be derived from it in a simple and natural way.
On the min-cost Traveling Salesman Problem with Drone Ha, Quang Minh; Deville, Yves; Pham, Quang Dung ...
Transportation research. Part C, Emerging technologies,
January 2018, 2018-01-00, Letnik:
86
Journal Article
Recenzirano
•We added a paragraph explaining the waiting costs based on the reviewer’s comment.•We fixed all the minor issues.•We have updated the latest references following the editor’s comments.•We prepared ...the paper following the editor’s comments.
Over the past few years, unmanned aerial vehicles (UAV), also known as drones, have been adopted as part of a new logistic method in the commercial sector called “last-mile delivery”. In this novel approach, they are deployed alongside trucks to deliver goods to customers to improve the quality of service and reduce the transportation cost. This approach gives rise to a new variant of the traveling salesman problem (TSP), called TSP with drone (TSP-D). A variant of this problem that aims to minimize the time at which truck and drone finish the service (or, in other words, to maximize the quality of service) was studied in the work of Murray and Chu (2015). In contrast, this paper considers a new variant of TSP-D in which the objective is to minimize operational costs including total transportation cost and one created by waste time a vehicle has to wait for the other. The problem is first formulated mathematically. Then, two algorithms are proposed for the solution. The first algorithm (TSP-LS) was adapted from the approach proposed by Murray and Chu (2015), in which an optimal TSP solution is converted to a feasible TSP-D solution by local searches. The second algorithm, a Greedy Randomized Adaptive Search Procedure (GRASP), is based on a new split procedure that optimally splits any TSP tour into a TSP-D solution. After a TSP-D solution has been generated, it is then improved through local search operators. Numerical results obtained on various instances of both objective functions with different sizes and characteristics are presented. The results show that GRASP outperforms TSP-LS in terms of solution quality under an acceptable running time.
The authors describe transmission of 2019-nCoV from a father, who had flown with his wife from Wuhan to Hanoi, to the son, who met his father and mother in central Vietnam and shared a hotel room ...with them for 3 days. The findings suggest that the incubation period in the son may have been 3 days or less.
3D concrete printing has recently become the subject of very rapidly growing research activities all over the world. An obstacle to develop 3D concrete has been the lack of tensile strength and ...therefore, limiting the printed component for structural application. This can be partly solved by designing fiber reinforced concrete or concrete with in-process embedded steel reinforcement. This article presents our first findings of using a steel cable reinforcing nozzle which can directly entrain continuous steel cable inside any extrudable mortar (e.g geopolymer), thus creating a hybrid reinforcement, that will improve flexural strength and ductility of the geopolymer composite. We have tested different cable reinforcements and compared their performance based on 4-point bending test. From experimental results, it was found that steel cables can provide a suitable reinforcement and hence improve the flexural strength of 3D printed concrete by 290%. Further research into optimal reinforcement placement, configuration and reinforcement material is recommended.
Finding the time-optimal parameterization of a given path subject to kinodynamic constraints is an essential component in many robotic theories and applications. The objective of this paper is to ...provide a general, fast, and robust implementation of this component. For this, we give a complete solution to the issue of dynamic singularities, which are the main cause of failure in existing implementations. We then present an open-source implementation of the algorithm in C++/Python and demonstrate its robustness and speed in various robotics settings.
To assess the role of in-flight transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), we investigated a cluster of cases among passengers on a 10-hour commercial flight. ...Affected persons were passengers, crew, and their close contacts. We traced 217 passengers and crew to their final destinations and interviewed, tested, and quarantined them. Among the 16 persons in whom SARS-CoV-2 infection was detected, 12 (75%) were passengers seated in business class along with the only symptomatic person (attack rate 62%). Seating proximity was strongly associated with increased infection risk (risk ratio 7.3, 95% CI 1.2-46.2). We found no strong evidence supporting alternative transmission scenarios. In-flight transmission that probably originated from 1 symptomatic passenger caused a large cluster of cases during a long flight. Guidelines for preventing SARS-CoV-2 infection among air passengers should consider individual passengers' risk for infection, the number of passengers traveling, and flight duration.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, ODKLJ, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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The effects of laser scan direction, part placement and inert gas flow velocity on the tensile strength of aluminium alloy, AlSi10Mg parts manufactured using Selective Laser Melting ...(SLM), were studied. The scan direction, either in the same or opposite direction to the gas flow was the primary factor of interest due to the possible laser-spattered powder-gas interactions leading to energy loss while scanning the powder bed. The effects of flow velocity on tensile strength had shown to be more significant than part placement. It was found that scanning against the gas flow resulted in better part quality, as quantified by a higher Ultimate Tensile Strength (UTS). This relationship was interpreted and discussed based on other observations, such as the quantity of accumulated spattered powder near the outlet, images recorded using high-speed cameras, SEM-EDS tests, etc. Hence, the results suggest a concrete modification to existing scanning strategies with respect to the inert gas flow in SLM to improve part quality.
Environmental protection has become a significant issue around the globe. The extensive use of renewable energy and green finance is considered as the solution to this dramatic issue, especially in ...the Covid-19 lockdown. To answer this demand, the present study examines the impact of green financial development such as green credit, green investment, and green securities along with corporate social responsibility (CSR) in reporting renewable energy investment based on evidence from an emerging economy. Economic growth was used as the control variable of the study. The data was gathered from the central bank and World Development Indicators (WDI) from 1976 to 2020. The error correction model (ECM) was used to test the nexus among the variables. The findings revealed that green credit, green investment, and green securities along with CSR reporting and economic growth have a significant positive nexus with renewable energy investment in the selected emerging economy. These outcomes are helpful for new arrivals to investigate this area in the future along with regulators who want to formulate policies related to green finance and renewable energy usage and investment in the context of emerging and developing countries.
Deep learning techniques for point cloud data have demonstrated great potentials in solving classical problems in 3D computer vision such as 3D object classification and segmentation. Several recent ...3D object classification methods have reported state-of-the-art performance on CAD model datasets such as ModelNet40 with high accuracy ( ~ 92\%). Despite such impressive results, in this paper, we argue that object classification is still a challenging task when objects are framed with real-world settings. To prove this, we introduce ScanObjectNN, a new real-world point cloud object dataset based on scanned indoor scene data. From our comprehensive benchmark, we show that our dataset poses great challenges to existing point cloud classification techniques as objects from real-world scans are often cluttered with background and/or are partial due to occlusions. We identify three key open problems for point cloud object classification, and propose new point cloud classification neural networks that achieve state-of-the-art performance on classifying objects with cluttered background. Our dataset and code are publicly available in our project page https://hkust-vgd.github.io/scanobjectnn/.
Spattering is an unavoidable phenomenon during Selective Laser Melting (SLM). The distinctively large and dark solidified molten particles have always been observed on the powder bed, leading to the ...potential deterioration of the final printed parts. In commercial SLM machines, inert gas flow is pumped into the chamber over the powder bed to remove spatter and other unwanted by-products such as metal vapour and plasma plumes. However, traces of spatter still remain on the powder bed and the regions close to the chamber outlet. In this work, the trajectories of spatter particles were tracked using the Discrete Phase Model (DPM). The continuous fluid domain was solved using three-dimensional Computational Fluid Dynamics (CFD) simulations. The entrainment effects on the hot ejections as well as the recoil pressure driven ejections were considered. In other words, it was assumed that the latter were also entrained by the metal vapour and laser plume. The final distribution characteristics downstream of the cross-flow (-x direction) were validated by an earlier experimental study. The results show that a gradually decreasing trend in terms of the spatter size and mass distributions were obtained from the simulations which showed good agreement with the earlier experiments. It was also shown that the trajectories of the spatter particles were governed mainly by their initial momentum when the inert gas flow velocity was increased. While complete removal of the spatter particles from the powder bed was not achieved, the presence of the inert gas flow aided in the transport of a majority of the particles further downstream giving a more even mass distribution. Also, in spite of not accounting for the complete multi-physics of the SLM process and the associated recoil pressure driven ejections, the proposed spatter transport simulations can be applied in future gas cross-flow optimisation designs for the effective removal of spatter over the powder bed in commercial SLM machines.
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•Entrainment driven spatter particles were injected into a CFD domain of inert gas cross-flow.•Good agreement was achieved between experiment and simulation results for size and mass distribution of spatter particles.•Presence of inert gas cross-flow resulted in a more even distribution of spatter mass.•Current simulations can be applied in gas flow optimisation studies for commercial SLM machines.
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