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A Three-Arm Current Comparator Digitally Assisted Bridge for the Comparison of Arbitrary Four Terminal-Pair Impedances
No full textHigh accuracy impedance measurements are performed with coaxial transformer bridges, whose conventional design allows the comparison of like impedances - pure resistors or capacitors. Here, we present a current comparator bridge suited for the measurement of impedances of arbitrary magnitudes and phase angles. The bridge has three arms, to connect the impedance under measurement and two calibrated standards. The bridge is digitally assisted and its operation is based on a polyphase digital sine wave synthesizer. To allow the measurement of midimpedance to low-impedance magnitudes, the bridge network and the balancing procedure are designed to approximate the four terminal-pair definition of the three impedance standards. The bridge has been extensively tested with conventional impedance standards and custom designed phase standards. The relative base accuracy is in the 10^−5 to 10^−6 range at kilohertz frequency
La facciata come cantiere
No full textL'intervento si propone di discutere e periodizzare il rapporto tra il concetto ideale di "facciata" di chiesa, così come è venuto definendosi nella storia della cultura architettonica tra il XII e il XVI secolo (Riccardo di San Vittore, Guglielmo Durando, Leon Battista Alberti, Francesco di Giorgio Martini), e la consistenza formale di quanto noi oggi comunemente chiamiamo "facciata", attraverso l'analisi di alcuni cantieri di particolare rilevanza nella storia dell'architettura occidentale (Saint-Denis, Santa Maria del Fiore, Duomo di Milano, San Petronio, cattedrale di Reggio Emilia). Il saggio si inserisce in una ricerca interdisciplinare sulle facciate e sui prospetti esterni degli edifici ecclesiali, con contributi - tra gli altri - di Vittorio Gregotti, Rafael Moneo, Aimaro Isola, Paolo Portoghesi, Cino Zucchi
Automotive composite components design and test: a vibration reduction oriented approach
No full textVehicle manufacturers initially relied on horsepower and speed performances as "standout qualities" to sell their products, but as time progressed, it became increasingly clear that customers were very much concerned about their comfort and safety while driving vehicles. In the late seventies, requirements for driver and passenger comfort were increased significantly, since-then, a large amount of effort has been invested into the vehicle improvement of noise and vibration reduction nevertheless enhancing the safety. Nowadays NVH performance is considered as a critical factor in the purchase decisions of many buyers. This work focusses on the development of a composite automotive door but, the methodology and materials applied could be extended for other panels, as there are several kinds in a typical vehicle whatever class or purpose it has. In the last years, furthermore, lightweight design imposed different design methodologies and materials like fiber reinforced plastics are becoming more and more used in the construction of vehicles for structural and non-structural parts. Reinforced plastics could perform higher performance and higher tailorable possibilities compared to conventional metallic materials, more and more composites are targeted toward structural applications involving combined dynamic, mechanical, thermal and hygral loading. Composite materials are principally preferred in such applications because of their advanced elastic properties and tailoring capability to individual design requirements. They also have the potential for incorporating significant and tailorable passive damping into the candidate structure. At the beginning, tests on two different viscoelastic materials are performed in order to identify mechanical characteristics necessary to provide valid data for numerical analysis of structures. ‘Oberst Beam Method' is the standard test method for measuring dynamic properties of materials (ASTM E756 - 05). Mechanical characteristics of damping materials have been determined for different temperatures (SAE J1637), frequencies and at different stage of aging (ISO 60068). Besides, low-velocity impact tests have been executed (ASTM 7136). After having post-processed the experimental data, the best material has been chosen. For each test, for each material and specimens topology a significant number of specimen has been tested for statistical purpose. ASTM 756 tests have been replicated virtually and to compare the results FRF Receptance, eigenfrequencies have been matched. Excellent correlation between simulation and experiment has been found. To evaluate the FEM virtual capabilities to reproduce and simulate the dynamic response of a real component and to validate correlation methodology, a first modal analysis has been carried out on a real automotive component. An innovative composite leaf spring is subjected to a free-free frequency response test, in which a series of accelerometers measures the response of the component to an excitation imposed by an electromechanical shaker. Moreover, LIPEZ methodology permitted to post-process data logged and to evaluate the characteristic vibrating modes of the structure. While the real component has been experimentally tested, FEM numerical simulation is reproduced. In the end, eigenfrequencies, eigenmodes (MAC) of experimental and virtual tests have been compared; excellent mode shape correlation has been found-out, errors on eigenfrequencies lower than 5%. Last part of this dissertation is focussed on the design, testing, and development of a damped door by means of interlaminar materials. The activity has been performed in two stages: undamped door testing and correlation with FEM and then manufacturing, testing and correlation of the damped solution. LIPEZ modal extraction algorithm has been used to extract modal parameters for the two artifacts, as well. The undamped door has been tested, real and virtual receptance, eigenfrequencies, eigenmodes computed and correlated. Damped material patches have been dimensioned and positioned considering a trade-off between added mass, costs and vibro-reduction capabilities. Modal analysis executed on the door have demonstrated that high damping capabilities have been achieved with limited drawbacks on weight increase (+5%) and inappreciable cost augmentation. In view of the scopes of the work, the results are deemed satisfactory
Cobalt-doped mesoporous carbon nanofibres as free-standing cathodes for lithium-oxygen batteries
No full textInterpolation-based learning as a mean to speed-up Bounded Model Checking
No full textIn this paper we introduce a technique to improve the efficiency of SAT calls in Bounded Model Checking (BMC) problems. The proposed technique is based on exploiting interpolation-based invariants as redundant constraints for BMC. Previous research addressed the issue using over-approximated state sets generated by BDD-based traversals. While a BDD engine could be considered as an external tool, interpolants are directly related to BMC problems, as they come from SAT-generated refutation proofs, so their role as a SAT-based earning is potentially higher. Our work aims at understanding whether and how interpolants could speed up BMC checks, as they represent constraints on forward and backward reachable states at given unrolling boundaries. Being this work preliminary, we do not address a tight integration between interpolant generation and exploitation. We thus clearly distinguish an interpolant generation (learning) phase and a subsequent interpolant exploitation phase in a BMC run. We experimentally evaluate costs, benefits, as well as invariant selection options, on a set of publicly available model checking problems
Educare al progetto, con passione
No full textLa formazione nel campo dell'architettura sta subendo un processo di revisione e aggiornamento che punta sul rinnovamento dei contenuti degli insegnamenti e delle esperienze progettuali, e su una maggiore interazione fra docenti e studenti. Il saggio elabora una riflessione su cosa si può e si deve insegnare oggi agli studenti di architettura, e cosa questi studenti dovrebbero apprendere per essere pronti a svolgere i nuovi mestieri dell'architetto
Electro-mechanical and electronic integrated harvester for shoes application
No full textEnergy harvesting allows making sensors or transmitters electrically autonomous. Several studies have been proposed but in most cases lack of electrical and mechanical parts integration and practical application purpose. Here, with the aim of supply a Bluetooth step-counter placed in the sole of a training shoe, a complete system is presented. It consists of a magnetoinductive transducer embedded with an electronic interface for power conditioning and exploits only the energy recovered by the impact on the ground. A 27 × 16 mm cylindrical device containing the transducer, the electronic interface, the step-counter electronics and the protective shell is developed. Energy recovery derives from the magnet free oscillation following the shoe impact. The proposed interface exploits pulse width modulation to perform transducer output rectification and to emulate optimal load impedance while charging a storage capacitor. Numerical and experimental analysis show effective optimal resistive load emulation and energy recovery gain of 2 compared to the standard AC-DC interface. Prototypes has been manufactured and tested. Considering a sample foot-step, energy recovery is 360 μJ against 400 μJ dissipated on optimal resistor. Mean energy recovery over a complete run is 644 μJ/foot-step. In spite of variable excitation, energy recovery at each foot-step is larger than the required and allows the step-counter transmitting the information
Shallow-dome structures and interaction between buckling and snap-through
No full textLightweight structures have the advantages of being ecological, virtuous, and beautiful to the eyes. The transparency and the fluent geometry that can be achieved with these structures, make them a reference point in modern structural engineering. Their design and production attain to many challenging fields in engineering such as the digital fabrication, the control and optimization theory, and the smart structures. However, their slenderness returns an high sensitivity to instability problems. Stability of equilibrium has a key role in structural engineering: the multiple nature and the vast set of influencing factors makes necessary a constant investigation and development of focused approaches. Above all, the sudden character that distinguish the instabilities requires a consistent effort to assure the safety of large and slender structures. Lastly, the interactive and dynamic essence of the instability demands a complex composition of solution schemes to treat it from a static point of view. Aim of this thesis is to study the interaction between the snap-through and the buckling and its effect on maximum load reduction, imperfection sensitivity, and especially on the variation in the structural behavior. Even if the dangerousness of the phenomenon is known, the problem requires a systematic treatment that concerns the study of the equilibrium paths. The path shape and characteristic provides crucial information about variation of inner forces regime, shallowness and slenderness effects, imperfections and connections influence. Hence, in the first part of the manuscript, a focused theoretical approach is devoted to the establish the describing and discriminating parameters. Additionally, a series of numerical simulations are presented to validate the formulations. Obtained results fulfill a first step in covering the gap between the physical knowledge of the problem and its actual scientific connotation. The results find an application in the stability of reticulated structures, commonly geometrically non-linear, but also a useful tool for the design and analysis of MEMS and micro-switchers. In this scale, the interaction phenomenon is too much often used for triggering a device without being completely grasped. The heinous data about mems collapse due to pull-in instability, about more than 40% of mems broke in this manner, is a reliable witness. In the structural engineering field, from the famous collapses of the 60's and 70's to the 2014 emblematic collapses of more than 100 reticulated domes in China caused by some exceptional snow storms. International organizations, such as the IASS, redacted new guideline for design, constructions and reliability of metal roofs, demonstrating a renewed interest in the interactive collapses. Among all, new methods for taking into account the imperfection sensitivity has been presented, based on probabilistic approaches. In the second part of the thesis, using the results obtained for a simple structures, the structural weaknesses and flaws are investigated. The results suggest that the methods adopted, commonly used for Fracture Mechanics, are useful for studying the local instability occurrence and consequences. For classical morphologies, such as diamatic and geodesic domes, the elastic local collapses of lateral and compressive buckling appears to be responsible for sharp load decreases. At the same time, for grid shells, the high stiffness connections required, proves to be a trigger to progressive and catastrophic collapses. Identification of snap-back phenomena in the equilibrium path of these structures are similar to the shell and cylinders collapses, where the intensity of flexural and axial regime of forces determines the catastrophic degree of the fail. The concept illustrated in this manuscript can be used for the safety assessment of lightweight structures and for optimization criteria for mems and micro-scale material structure
Production of PHA from whey by indigenous microflora and activated sludge: preliminary investigation
No full textMilk whey was employed to produce polyhydroxyalcanoates (PHA), using as inoculum either whey indigenous microflora or enriched activated sludge deriving from a dairy wastewater treatment plant. The aim of the present study was to test the capability of mixed consortia to produce PHA directly from lactose. The experimental runs demonstrated that milk whey lactose can constitute a cheap carbon source to synthesize PHA in fermentations where the microorganisms are represented by indigenous microorganisms or enriched activated sludge: this result constitutes an evident advantage since whey pre-treatment and aseptic conditions are not required. Tests with commercial whey powder (Molkolac®) and on raw milk whey showed a PHA production around 10 % (by weight on dry biomass)