ENIDH – EEM - Artigo Científico
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- Lumped Model for CMOS Interconnections linesPublication . Assunção, Mário; Santos, Pedro; Freire, João CostaA study on CMOS interconnections lines up to 40GHz is presented. The lines propagation constant γ and impedance characteristic Z0 are obtained from the measured S parameters up to 40 GHz, on test structures with different lengths, with or without metal ground plane. The values of the well-defined transmission line RLGC lumped model elements per unit length for each measured test interconnection lines are obtained. Although, the RLGC elements values are frequency dependent, we show that using constant values in a wideband, from 5 GHz to 40 GHz, the interconnections model is in good agreement with the experimental S parameters, validating the modelling technique. As expected, for shorter lines, one RLGC section is adequate to attain a good agreement with measured values. However, for longer lines, the number of RLGC sections must be increased. For low frequencies the accuracy is better for interconnections lines with metal ground plane. However, for high frequencies (f > 15 GHz) the accuracy is good for both type of interconnection lines.
- Voltage Spikes in Integrated CMOS Buck DC-DC Converters: Analysis for Resonant and Hard Switching TopologiesPublication . Rocha, José F. da; Santos, Marcelino B. dos; Costa, J. M. DoresIn an integrated DC-DC converter, voltage spikes are generated during the commutation of the power switches and they may cause the device malfunction if its magnitude is excessive. That occurs when, targeting high efficiency, very fast switching is implemented in a low voltage CMOS process, with higher impact when the converter output current is high. These conditions that cause spikes of significant magnitude, in the order of Volts, are common in modern DC-DC converters for portable equipment powered by batteries. Magnitude limitation of voltage spikes in hard switching converters is a hot research topic and the known solutions are surveyed here. Resonant switches topologies are frequently mentioned as an alternative to overcome voltage spikes in a DC-DC converter, but generally this solution is dismissed in integrated implementations. This paper investigates if the resonant topologies are an effective solution to overcome voltage spikes during the switches commutation in a buck converter. Two buck DC-DC converters using resonant switches are analyzed, namely a QR-ZCS topology and a QSW- ZVS topology. The analysis proceeds in two steps that can be easily applied to other resonant converters. Results show that resonant DC-DC converters also generate voltage spikes which magnitude is sometimes higher than that generated in a hard switching converter, and is superimposed to the overvoltage occurring in the resonant phase. This is an important result since resonant, or soft switching, converters are occasionally mistakenly mentioned as free of voltage spikes.
- 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.
- Modular buck-boost transformerless grid-tied inverter for low voltage solar panelsPublication . Nunes, Hugo; Pimenta, Nelson; Fernandes, Luís; Chaves, Paulo; Costa, J. M. DoresThis paper deals with a transformerless AC module for low DC voltage output photovoltaic solar panels. The DC to AC converter was designed to be a single-phase grid-tied inverter, and is based on a buck-boost topology with current mode control to shape the current and to adjust delivered power. The common ground node prevents voltage fluctuations and increases people´s safety. Low power inverters tend to be less efficient than larger ones and are prone to higher harmonic distortion. To overcome these drawbacks, special care was taken in designing the power stage, and harmonic compensation was included in the control stage. The inverter has MPPT capability and operates with unity power factor. Inverter´s operation is analysed, and some simulation results are presented. A 220Wp two-stage single-phase AC module prototype, with 240V and 50Hz sine-wave output, was build and connected to a low voltage photovoltaic solar panel. Experimental results for this plug-and-play converter are here presented.
- The Finitistic Consistency of Heck’s Predicative Fregean SystemPublication . Cruz-Filipe, L.; Ferreira, FernandoFrege’s theory is inconsistent (Russell’s paradox). However, the predicative version of Frege’s system is consistent. This was proved by Richard Heck in 1996 using a model-theoretic argument. In this paper, we give a finitistic proof of this consistency result. As a consequence, Heck’s predicative theory is rather weak (as was suspected).
- On the assessment of fatigue life of marine diesel engine crankshaftsPublication . Fonte, M.; Duarte, P.; Anes, V.; Freitas, M.; Reis, L.The fatigue strength and its correct assessment play an important role in design and maintenance of marine crankshafts to obtain operational safety and reliability. Crankshafts are under alternating bending on crankpins and rotating bending combined with torsion on main journals, which mostly are responsible for fatigue failure. The commercial management success substantially depends on the main engine in service and of its design crankshaft, in particular. The crankshaft design strictly follows the rules of classification societies. The present study provides an overview on the assessment of fatigue life of marine engine crankshafts and its maintenance taking into account the design improving in the last decades, considering that accurate estimation of fatigue life is very important to ensure safety of components and its reliability. An example of a semi-built crankshaft failure is also presented and the probable root case of damage, and at the end some final remarks are presented.
- Layerwise mixed models for analysis of multilayered piezoelectric composite plates using least-squares formulationPublication . Moleiro, F.; Mota Soares, C.M.; Mota Soares, C.A.; Reddy, J.N.This work provides an assessment of layerwise mixed models using least-squares formulation for the coupled electromechanical static analysis of multilayered plates. In agreement with three-dimensional (3D) exact solutions, due to compatibility and equilibrium conditions at the layers interfaces, certain mechanical and electrical variables must fulfill interlaminar C0 continuity, namely: displacements, in-plane strains, transverse stresses, electric potential, in-plane electric field components and transverse electric displacement (if no potential is imposed between layers). Hence, two layerwise mixed least-squares models are here investigated, with two different sets of chosen independent variables: Model A, developed earlier, fulfills a priori the interlaminar C0 continuity of all those aforementioned variables, taken as independent variables; Model B, here newly developed, rather reduces the number of independent variables, but also fulfills a priori the interlaminar C0 continuity of displacements, transverse stresses, electric potential and transverse electric displacement, taken as independent variables. The predictive capabilities of both models are assessed by comparison with 3D exact solutions, considering multilayered piezoelectric composite plates of different aspect ratios, under an applied transverse load or surface potential. It is shown that both models are able to predict an accurate quasi-3D description of the static electromechanical analysis of multilayered plates for all aspect ratios.
- Welding assessment of a damaged crane pedestal of a container shipPublication . Fonte, M.; Freitas, M.M.; Li, B.; Duarte, P.; Reis, L.A structural integrity assessment of a damaged crane pedestal/column of a container ship is presented. Crane cabins and pedestal/columns are subjected to fatigue coupled with sudden overloads during cargo operations and corrosion effects due to sea environment. Significant number of surface cracks was detected around the pedestal/column, at inner and outside of the crane foundation, close to the main weld seam and, in consequence, the ship-owner ordered a survey. The main weld seam and the sites where cracks were found are evaluated in the present study. For the purpose, the sample material (hot rolled steel plate and upper ring) of the pedestal was provided by the shipyard and macro and micrographics were observed. Results did not show any cracks, although they have been found by the NDT technical services of the shipyard which have decided to remove them by the grinding process. The weld seam did not also show relevant defects, whereby the replacing of the pedestal/column by a new one would not had been necessary. Regardless of some occasional overloads, the surface cracks found on the pedestal/column could be a consequence of the normal operation conditions of the crane during the last 5 years and also due to poor maintenance.
- A technique to reduce on-wafer measurements uncertainty on multiline method for CMOS Transmission Lines characterizationPublication . Assunção, Mário; Santos, P.; Freire, J. C.An improvement to the L-L method for Si monolithic Transmission Line (TL) parameter extraction is presented. The propagation constant γ and characteristic impedance Z0 are obtained from cascaded ABCD matrices using on-wafer measurements up to 40 GHz. It is shown that the L-L method matrices processing is strongly dependent on the access variations as different pairs of measurements lead to different extracted TL parameters. Associating three different TLs lengths test structures measurements matrices, which theoretically point to an identity matrix, a technique is proposed to obtain the best set of three measurements with the most similar accesses. The deviation from the ideal identity matrix indicates the access variations. The suggested technique was implemented with TL structures fabricated on a 130 nm CMOS process. When using the best set of three different TL length measurements, the extracted γ and Z0 parameters are almost independent on the TL pair used.
- Desafios actuais e futuros para o ensino náuticoPublication . Costa, José Dores