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- Adiabatic radial perturbations of relativistic stars: Analytic solutions to an old problemPublication . Luz, Paulo; Carloni, SanteWe present a new system of equations that fully characterizes adiabatic, radial perturbations of perfect fluid stars within the theory of general relativity. The properties of the system are discussed, and, provided that the equilibrium spacetime verifies some general regularity conditions, analytical solutions for the perturbation variables are found. As illustrative examples, the results are applied to study perturbations of selected classical exact spacetimes, and the first oscillation eigenfrequencies are computed. Exploiting the new formalism, we derive an upper bound for the maximum compactness of stable, perfect fluid stars, which is equation-of-state agnostic and significantly smaller than the Buchdahl bound.
- Análise Qualitativa e Quantitativa do Cluster do Mar PortuguêsPublication . Simões, Abel; Ferreira, Ana Maria; Salvador, Regina; Soares, Carlos GuedesReconhecendo-se a importância de um Cluster do Mar para Portugal, procura-se identificar e analisar as principais relações intersetoriais que se processam no seu interior. Assim, na primeira parte é feita uma análise de carácter mais qualitativo, através da aplicação de inquéritos a empresas de transporte marítimo, estaleiros navais, portos, associações empresariais e instituições de ensino e investigação. Na segunda parte, utilizando a análise de input-output procede-se à quantificação das ligações existentes entre setores do Cluster marítimo português, ao cálculo dos coeficientes de interdependência e apresentam-se alguns dos efeitos multiplicadores das atividades marítimas. Algumas debilidades estruturais, que são comuns a outras áreas, como sejam a fraca cooperação e competição entre setores económicos e incipientes relações intersetoriais, refletem-se ao nível deste Cluster como uma das suas principais fragilidades, a ponto de comprometer o reconhecimento da sua existência na economia marítima nacional.
- Analysis and Design of Current Mode Class-D Power Amplifiers With Finite Feeding InductorsPublication . Silva-Pereira, M.; Assunção, Mário; Vaz, João CaldinhasCurrent-mode class-D (CMCD) power amplifiers (PAs) may not achieve a switching efficiency as good as class-E PAs, but they require fewer inductances and deliver almost five times more power for the same load and supply voltage when compared with the ideal push–pull class-E PA. Furthermore, CMCD PAs can easily use bondwires as the only circuit inductors, a precious feature that Internet-of-Things (IoT) wireless interfaces can exploit. For this reason, we analyze the voltage and current waveforms in CMCD PAs without assuming infinite value feeding inductors (i.e., without choke inductors). It is shown that the current stress on the switching device is significantly higher than in the conventional topology, especially in bondwire-based designs where relatively small feeding induc tances are presented. Finally, we propose a design procedure for a given feeding inductance that maximizes energy efficiency while delivering a compact-size solution. A 2.4-GHz PA capable of delivering 19.8 dBm to a 100-Ω differential load with a power added-efficiency (PAE) of 58% is presented. Three-dimensional electromagnetic (3-D EM) simulations combined with postlayout simulations were used to evaluate the PA performance in a quad-flat nonlead (QFN)-type package. The circuit is designed in a 0.13-μm process and only occupies a silicon area of 0.025 mm2.
- Architectural Design for Heartbeat Detection Circuits using Verilog-A Behavioral ModelingPublication . Vieira, R.; Passos, F.; Póvoa, R.; Martins, R.; Horta, N.; Guilherme, J.; Lourenço, N.This work presents a study of two analog frontend circuit architectures for heartbeat detection. Both circuits present an amplification block as the first stage, followed by a band-pass filter. In the first, the heartbeat detection is done using an adaptive threshold based on pulse-width, whereas the heartbeat detection in the second is done using a sample and hold to find the maximum and minimum peak of each beating. Both architectures are modeled in Verilog-A and simulated using real-world ECG signals with different characteristics. This work studies possible fundamental analog circuit blocks suitable for wearable implementation. It evaluates critical performances requirements from the analysis of the behavior simulations. It was verified that the first circuit can properly detect heartbeats as long as the input-referred noise is below 21 μV, whereas the second one ensures it until 30 μV. The low cutoff frequency can be approximately 10 Hz without compromising the signal’s peaks, which means that these specifications can be relaxed substantially compared to systems intended to reconstruct the signal accurately.
- Buckling and nonlinear response of functionally graded plates under thermo-mechanical loadingPublication . Moita, José S.; Araújo, Aurélio L.; Correia, Victor Franco; Soares, C. M. Mota; Herskovits, Josén this work, a formulation for buckling and geometrically nonlinear analysis of functionally graded plates under thermo-mechanical loading is presented. The influence of high temperature environment on deflections due to mechanical loading as well the deflections produced by thermal loads acting alone or combined with mechanical loads is analysed. The implemented finite element model is based on a non-conforming triangular flat plate/shell element, with three nodes and eight degrees of freedom per node, associated with a higher order shear deformation theory. The solutions of some illustrative examples are evaluated involving the variation of volume fractions of constituent materials, temperature range and material combinations, which have significant impact on the deflections of plates. The numerical results obtained with the present model are presented and compared with alternative solutions.
- Coefficient of Voltage Energy EfficiencyPublication . Gnaciński, P.; Mindykowski, J.; Pepliński, M.; Tarasiuk, T.; Costa, J. D.; Assunção, M.; Silveira, L.; Zakharchenko, V.; Drankova, A.; Mukha, M.; Xu, XThe reduction of greenhouse gas emissions is a major contemporary challenge. This has prompted the requirements concerning energy efficiency for ships, among other things. Improvement in eficiency of ship operations could be achieved by reducing unnecessary power consumption by induction motors. Specifically, the occurrence of power quality disturbances, such as frequency and voltage deviations, voltage unbalances, and voltage harmonics, can cause an extraordinary increase in the power losses occurring in induction motors, as well as an unnecessary increase in the output power. Furthermore, excessive power quality disturbances are often interconnected with failures of on-board equipment, and in extreme cases, these may even pose a threat to safety at sea. Consequently, strict power quality monitoring of on-board microgrids can also contribute to improving safety while afloat. In this study, a dedicated tool for power quality monitoring is proposed, namely the coeficient of voltage energy efficiency, which has a value proportional to the power losses occurring in induction motors under power quality disturbances.
- DeepPlacer: A Custom Integrated OpAmp Placement Tool using Deep ModelsPublication . Gusmão, António; Póvoa, Ricardo; Horta, Nuno; Lourenço, Nuno; Martins, RicardoMechanisms towards the automatic analog integrated circuit layout design have been an intensive research topic in the past few decades. Still, the industrial environment has no automatic approach established. The advances of machine learning applications in electronic design automation come with the promise to change this reality. This paper proposes a deep learning generative model for the placement ‘‘optimization’’ of analog integrated circuit basic blocks. The model behaves as an argmin operator for the placement cost function and can provide placement solutions instantly. Moreover, the model can be fed with unlabeled data, greatly facilitating data collection. A generic and innovative circuits’ representation at the network’s input layer is proposed, encoding the devices’ dimensions, connectivity, and topological constraints. Besides, the randomness found in generative models is embedded directly into the feature vector, as the order of the features per device is shuffled in the input vector. Shuffling the order of the devices’ features in the input not only brings multi-modality but also solves a generalization problem, as there is not any natural order defined to place devices in the feature vector. As a proof of concept, a deep artificial neural network capable of proposing different placement solutions, in less than 150 ms each, for six amplifier topologies and, in multiple technology nodes ranging from 350 nm down to 65 nm, is demonstrated. DeepPlacer was capable of producing correct solutions for topologies and technology nodes not present in the training set, showing good generalization while not hindering circuit performance due to the placement
- Desafios actuais e futuros para o ensino náuticoPublication . Costa, José Dores
- Enhanced AutomaticWildfire Detection System Using Big Data and EfficientNetsPublication . Fernandes, Armando; Utkin, Andrei; Chaves, PauloPrevious works have shown the effectiveness of EfficientNet—a convolutional neural network built upon the concept of compound scaling—in automatically detecting smoke plumes at a distance of several kilometres in visible camera images. Building on these results, we have created enhanced EfficientNet models capable of precisely identifying the smoke location due to the introduction of a mosaic-like output and achieving extremely reduced false positive percentages due to using partial AUROC and applying class imbalance. Our EfficientNets beat InceptionV3 and MobileNetV2 in the same dataset and achieved a true detection percentage of 89.2% and a false positive percentage of only 0.306% across a test set with 17,023 images. The complete dataset used in this study contains 26,204 smoke and 51,075 non-smoke images. This makes it one of the largest, if not the most extensive, datasets reported in the scientific literature for smoke plume imagery. So, the achieved percentages are not only among the best reported for this application but are also among the most reliable due to the extent and representativeness of the dataset.
- Extraction Optimization and Structural and Thermal Characterization of the Antimicrobial Abietane 7α-Acetoxy-6βhydroxyroyleanonePublication . Bernardes, C.; Garcia, C.; Pereira, F.; Mota, J.; Pereira, P.; Cebola, M.J.; Reis, C.; Correia, I.; Piedade, M.F.; Piedade, M.The abietane 7α-acetoxy-6β-hydroxyroyleanone (AHR), obtained from plant extracts, is an attractive lead for drug development, given its known antimicrobial properties. Two basic requirements to establish any compound as a new drug are the development of a convenient extraction process and the characterization of its structural and thermal properties. In this work seven different methods were tested to optimize the extraction of AHR from Plectranthus grandidentatus. Supercritical fluid extraction (SFE) proved to be the method of choice, delivering an amount of AHR (57.351 μg·mg-1) approximately six times higher than the second best method (maceration in acetone; 9.77 μg·mg-1). Single crystal X-ray diffraction analysis of the ARH molecular and crystal structure carried out at 167 ± 2 K and 296 ± 2 K showed only a single phase, here dubbed form III (orthorhombic space group P21212), at those temperatures. The presence of two other polymorphs above room temperature was, however, evidenced by differential scanning calorimetry (DSC). The three forms are enantiotropically related, with the form III → form II and form II → form I transitions occurring at 333.5 ± 1.6 K and 352.0 ± 1.6 K, respectively. The fact that the transitions are reversible suggests that polymorphism is not likely to be an issue in the development pharmaceutical formulations based on ARH. DSC experiments also showed that the compound decomposes on melting at 500.8 ± 0.8 K. Melting should therefore be avoided if, for example, strategies to improve solubility based on the production of glassy materials or solid dispersions are considered.