| 1. |
- Anagnostopoulos, George, et al.
(författare)
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Strain Engineering in Highly Wrinkled CVD Graphene/Epoxy Systems
- 2018
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Ingår i: ACS Applied Materials & Interfaces. - : American Chemical Society (ACS). - 1944-8252 .- 1944-8244. ; 10:49, s. 43192-43202
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Tidskriftsartikel (refereegranskat)abstract
- Chemical vapor deposition (CVD) is regarded as a promising fabrication method for the automated, large-scale, production of graphene and other two-dimensional materials. However, its full commercial exploitation is limited by the presence of structural imperfections such as folds, wrinkles, and even cracks that downgrade its physical and mechanical properties. For example, as shown here by means of Raman spectroscopy, the stress transfer from an epoxy matrix to CVD graphene is on average 30% of that of exfoliated monolayer graphene of over 10 μm in dimensions. However, in terms of electrical response, the situation is reversed; the resistance has been found here to decrease by the imposition of mechanical deformation possibly due to the opening up of the structure and the associated increase of electron mobility. This finding paves the way for employing CVD graphene/epoxy composites or coatings as conductive "networks" or bridges in cases for which the conductivity needs to be increased or at least retained when the system is under deformation. The tuning/control of such systems and their operative limitations are discussed here.
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| 2. |
- Alur, Rajeev, et al.
(författare)
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Compositional Modeling and Analysis of Multi-Hop Control Networks
- 2011
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Ingår i: IEEE Transactions on Automatic Control. - 0018-9286 .- 1558-2523. ; 56:10, s. 2345-2357
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Tidskriftsartikel (refereegranskat)abstract
- We propose a mathematical framework for modeling and analyzing multi-hop control networks designed for systems consisting of multiple control loops closed over a multi-hop (wireless) communication network. We separate control, topology, routing, and scheduling and propose formal syntax and semantics for the dynamics of the composed system, providing an explicit translation of multi-hop control networks to switched systems. We propose formal models for analyzing robustness of multi-hop control networks, where data is exchanged through a multi-hop communication network subject to disruptions. When communication disruptions are long, compared to the speed of the control system, we propose to model them as permanent link failures. We show that the complexity of analyzing such failures is NP-hard, and discuss a way to overcome this limitation for practical cases using compositional analysis. For typical packet transmission errors, we propose a transient error model where links fail for one time slot independently of the past and of other links. We provide sufficient conditions for almost sure stability in presence of transient link failures, and give efficient decision procedures. We deal with errors that have random time span and show that, under some conditions, the permanent failure model can be used as a reliable abstraction. Our approach is compositional, namely it addresses the problem of designing scalable scheduling and routing policies for multiple control loops closed on the same multi-hop control network. We describe how the translation of multi-hop control networks to switched systems can be automated, and use it to solve control and networking co-design challenges in some representative examples, and to propose a scheduling solution in a mineral floatation control problem that can be implemented on a time triggered communication protocols for wireless networks.
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| 3. |
- Arun, K. G., et al.
(författare)
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New horizons for fundamental physics with LISA
- 2022
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Ingår i: Living Reviews in Relativity. - : Springer Science and Business Media LLC. - 1433-8351 .- 2367-3613. ; 25:1
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Forskningsöversikt (refereegranskat)abstract
- The Laser Interferometer Space Antenna (LISA) has the potential to reveal wonders about the fundamental theory of nature at play in the extreme gravity regime, where the gravitational interaction is both strong and dynamical. In this white paper, the Fundamental Physics Working Group of the LISA Consortium summarizes the current topics in fundamental physics where LISA observations of gravitational waves can be expected to provide key input. We provide the briefest of reviews to then delineate avenues for future research directions and to discuss connections between this working group, other working groups and the consortium work package teams. These connections must be developed for LISA to live up to its science potential in these areas.
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| 4. |
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| 5. |
- Barack, Leor, et al.
(författare)
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Black holes, gravitational waves and fundamental physics : a roadmap
- 2019
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Ingår i: Classical and quantum gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 36:14
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Forskningsöversikt (refereegranskat)abstract
- The grand challenges of contemporary fundamental physics dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions. The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature. The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress. This write-up is an initiative taken within the framework of the European Action on 'Black holes, Gravitational waves and Fundamental Physics'.
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| 6. |
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| 7. |
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| 8. |
- Speranzon, Alberto, 1975-
(författare)
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Coordination, Consensus and Communication in Multi-robot Control Systems
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Analysis, design and implementation of cooperative control strategies for multi-robot systems under communication constraints is the topic of this thesis. Motivated by a rapidly growing number of applications with networked robots and other vehicles, fundamental limits on the achievable collaborative behavior are studied for large teams of autonomous agents. In particular, a problem is researched in detail in which the group of agents is supposed to agree on a common state without any centralized coordination. Due to the dynamics of the individual agents and their varying connectivity, this problemis an extension of the classical consensus problemin computer science. It captures a crucial component of many desirable features of multi-robot systems, such as formation, flocking, rendezvous, synchronizing and covering. Analytical bounds on the convergence rate to consensus are derived for several systemconfigurations. It is shown that static communication networks that exhibit particular symmetries yield slow convergence, if the connectivity of each agent does not scale with the total number of agents. On the other hand, some randomly varying networks allow fast convergence even if the connectivity is low. It is furthermore argued that if the data being exchanged between the agents are quantized, it may heavily degrade the performance. The extent to which certain quantization schemes are more suitable than others is quantified through relations between the number of agents and the required total network bit rate. The design of distributed coordination and estimation schemes based on the consensus algorithm is presented. A receding horizon coordination strategy utilizing subgradient optimization is developed. Robustness and implementation aspects are discussed. A new collaborative estimation method is also proposed. The implementation of multi-robot control systems is difficult due to the high systemcomplexity. In the final part of this thesis, a hierarchical control architecture appropriate for a class of coordination tasks is therefore suggested. It allows a formal verification of the correctness of the implemented control algorithms.
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| 9. |
- Stamatakis, George, et al.
(författare)
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Autonomous Maintenance in IoT Networks via AoI-driven Deep Reinforcement Learning
- 2021
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Ingår i: IEEE CONFERENCE ON COMPUTER COMMUNICATIONS WORKSHOPS (IEEE INFOCOM WKSHPS 2021). - : IEEE. - 9781665404433 - 9781665447140
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Konferensbidrag (refereegranskat)abstract
- Internet of Things (IoT) with its growing number of deployed devices and applications raises significant challenges for network maintenance procedures. In this work, we formulate a problem of autonomous maintenance in IoT networks as a Partially Observable Markov Decision Process. Subsequently, we utilize Deep Reinforcement Learning algorithms (DRL) to train agents that decide if a maintenance procedure is in order or not and, in the former case, the proper type of maintenance needed. To avoid wasting the scarce resources of IoT networks we utilize the Age of Information (AoI) metric as a reward signal for the training of the smart agents. AoI captures the freshness of the sensory data which are transmitted by the IoT sensors as part of their normal service provision. Numerical results indicate that AoI integrates enough information about the past and present states of the system to be successfully used in the training of smart agents for the autonomous maintenance of the network.
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| 10. |
- Stamatakis, George, et al.
(författare)
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Control of Status Updates for Energy Harvesting Devices that Monitor Processes with Alarms
- 2019
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Ingår i: 2019 IEEE GLOBECOM WORKSHOPS (GC WKSHPS). - : IEEE. - 9781728109602
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Konferensbidrag (refereegranskat)abstract
- In this work, we derive optimal transmission policies for an energy harvesting status update system. The system monitors a stochastic process which can be in one of two states of operation, a normal state or an alarm state. We capture the freshness of status updates for each state of the stochastic process by using a separate Age of Information (AoI) variable for each state and by extending the definition of AoI to account for the stochastic process state changes. We assume that the demand for status updates is higher when the stochastic process is in the alarm state and utilize a transition cost function that increases non-linearly with AoI when the stochastic process is in the alarm state and linearly otherwise. We formulate the problem at hand as a Markov Decision Process, evaluate numerically the derived policies and illustrate their effectiveness for reserving energy in anticipation of future alarm states.
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