Proposal of a new simplified coulomb friction model applied to electrohydraulic servomechanisms

The design of electro-hydraulic servomechanisms characterized by high precision requirements generally needs adequate knowledge of its characteristics, and, in particular, of nonlinear phenomena. Among these, Coulomb's frictional forces acting on the mechanical elements in relative motion are critical to guarantee an implementation capable of respecting the accuracy requirements. The correct evaluation of this phenomenon allows understanding the behaviour of the physical system considered, to estimate its performance by implementing it in a simulation environment, and to design new devices taking into account the relative constraints. Accurate modelling and simulation of the considered system generally imply the use of high order dynamic models (typically, of second-order nonlinear or higher). However, under certain conditions, it is possible (and advisable) to simplify the mathematical structure of the numerical model, degrading it to a simple first-order, reducing its complexity and computational cost and, nevertheless, still obtaining results comparable with higher-order models. In this paper, the authors propose a new computational model capable of being implemented within these degraded numerical models, allowing them to simulate the main effects due to dry frictions (Coulomb's model). This first-order dynamic model is compared with the corresponding second-order ones to evaluate their performances in different scenarios

Integration of rule-based ‘Expert Systems’ on RPAS capable of specific category operations within the U-space: An original mitigation strategy for operational safety risks

The use of RPAS for civil purposes is spreading across Europe and worldwide; Aviation Authorities are working to layout regulations to assure a safe and secure integration of RPAS with manned aircraft across both controlled and uncontrolled (below 500 Feet of altitude) airspace. Following the identification of a selection of safety risks potentially associated to RPAS Specific Category of operations, an original strategy of risks mitigation focused on rule-based ‘Expert Systems’, has been conceived and it is discussed in this work. The article recalls the main components of rule-based ‘Expert Systems’ that is the knowledge basis and the rules to instruct the ‘Expert system’. Then the work describes the implementation of the rules as statements derived from a safety risk matrix associated to RPAS capable of performing Specific Category operations within the U-space. Finally, the idea of integrating the ‘Expert System’ as a software module within RPAS functional architecture is presented and discussed. Such solution is deemed to be a valuable novelty for future implementations of advanced RPAS autopilots capable of recognizing and solving in flight/on ground operational safety risks in such a way to speed up the integration of RPAS into not segregated airspace and their market development

Electrohydraulic actuators affected by multiple failures: Proposal of an alternative model-based prognostic paradigm

Onboard electrohydraulic actuator (EHA) applied to primary and secondary flight command, and in particular the servovalves (SVs) regulating their hydraulic power, are complex devices and can fail in several ways: servovalves are critical components of the hydraulic servos and their correct operation is mandatory to ensure the proper functioning of the controlled servosystem. For this reason, a continuous monitor is typically performed to detect a servovalve loss of operation, but this monitor falls short of recognizing other malfunctionings. Often, a progressive degradation of a servovalve occurs, which does not initially create an unacceptable behavior, but eventually leads to a condition in which the servovalve, and hence the whole servoactuator operation, is impaired. Developing a prognostic algorithm able to identify the precursors of a servovalve failure and its degradation pattern is thus beneficial for anticipating the incoming failure and alerting the maintenance crew such to properly schedule the servovalve replacement. This avoids a servovalve failure in service, thereby ensuring improved equipment availability and minimizing the impacts onto the logistic line. To this purpose, authors propose a new model-based fault detection and identification (FDI) technique able to perform an early detection of two of the most common types of SV progressive failures (dry friction acting on servovalve spool and contamination of the first stage filter). The robustness of the proposed technique has been assessed through a simulation test environment, built on the purpose. Such simulation has demonstrated that the methodology has adequate robustness; also, the ability to early identify an eventual malfunctioning has been proved with low risk of missed failures or false positives

A first-order lumped parameters model of electrohydraulic actuators for low-inertia rotating systems with dry friction

In aerospace engineering, there are several control systems affected by dry friction, which are characterized by low inertia and high working frequencies. For these systems, it is possible to use a downgraded, first-order dynamic model to represent their behaviour properly without run into numerical problems that would be harmful for the solution itself and would require high computational power to be solved, which means more weight, costs, and complexity. Yet, the effect of dry friction is still possible to be accounted for accurately using a new algorithm based on the Coulomb friction model applied to the downgraded, first-order dynamic model. In this paper, the degraded first-order model is applied to an electrohydraulic servomechanism with its PID control unit, hydraulic motor, electrohydraulic servo-valve, and applied load. These components represent a classic airplane actuator system. The downgraded model will be compared to the second-order one focusing on the pros and cons of the reduction process with a focus on the effect of dry friction for reversible and irreversible actuators

VLTI/PIONIER images the Achernar disk swell

Context. The mechanism of disk formation around fast-rotating Be stars is not well understood. In particular, it is not clear which mechanisms operate, in addition to fast rotation, to produce the observed variable ejection of matter. The star Achernar is a privileged laboratory to probe these additional mechanisms because it is close, presents B-Be phase variations on timescales ranging from 6 yr to 15 yr, a companion star was discovered around it, and probably presents a polar wind or jet. Aims. Despite all these previous studies, the disk around Achernar was never directly imaged. Therefore we seek to produce an image of the photosphere and close environment of the star. Methods. We used infrared long-baseline interferometry with the PIONIER/VLTI instrument to produce reconstructed images of the photosphere and close environment of the star over four years of observations. To study the disk formation, we compared the observations and reconstructed images to previously computed models of both the stellar photosphere alone (normal B phase) and the star presenting a circumstellar disk (Be phase). Results. The observations taken in 2011 and 2012, during the quiescent phase of Achernar, do not exhibit a disk at the detection limit of the instrument. In 2014, on the other hand, a disk was already formed and our reconstructed image reveals an extended H-band continuum excess flux. Our results from interferometric imaging are also supported by several H-alpha line profiles showing that Achernar started an emission-line phase sometime in the beginning of 2013. The analysis of our reconstructed images shows that the 2014 near-IR flux extends to 1.7 - 2.3 equatorial radii. Our model-independent size estimation of the H-band continuum contribution is compatible with the presence of a circumstellar disk, which is in good agreement with predictions from Be-disk models

Evaluations on hydrogen fuel cells as a source of energy for specific operations category civil RPAS systems

This paper is on the evaluation of hydrogen fuel cells as a mean to enhance RPAS systems performances in terms of reachable range and endurance to integrate them into controlled airspaces operatively and safely. Main steps to size a fuel cell system to feed electrical motors of a fixed wing RPAS capable of specific operations category are described in this article. Then, a more extensive parametric model of a fuel cell power line based on operative and safety requirements for medium range/medium endurance RPAS systems is presented and discussed

High-spin states and band terminations in v 49

High-spin states in 49 V have been studied through the 28 Si(28 Si, α3p) reaction using the EUROBALL γ-ray detector array. The 49 V level scheme has been extended up to 13.1 MeV including 21 new states. Both negative and positive parity states have been interpreted in the framework of theShell Model. The 27/2− and the 31/2+ band termination states have been observed in agreement with theoretical predictions.Fil: Rodrigues Ferreira Maltez, Dario Pablo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigación y Aplicaciones No Nucleares. Gerencia Física (Centro Atómico Constituyentes). Proyecto Tandar; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; ArgentinaFil: Hojman, Daniel Leonardo. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigación y Aplicaciones No Nucleares. Gerencia Física (Centro Atómico Constituyentes). Proyecto Tandar; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Lenzi, Silvia M.. Istituto Nazionale Di Fisica Nucleare.; Italia. Università di Padova; ItaliaFil: Cardona, Maria Angelica. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigación y Aplicaciones No Nucleares. Gerencia Física (Centro Atómico Constituyentes). Proyecto Tandar; Argentina. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; ArgentinaFil: Fernea, Enrico. Università di Padova; Italia. Istituto Nazionale Di Fisica Nucleare.; ItaliaFil: Axiotis, M.. Istituto Nazionale Di Fisica Nucleare.; ItaliaFil: Beck, C.. Université de Strasbourg; Francia. Centre National de la Recherche Scientifique; FranciaFil: Bednarczyk, P.. Polish Academy of Sciences; ArgentinaFil: Bizzetti, P. G.. Università di Padova; Italia. Istituto Nazionale Di Fisica Nucleare.; ItaliaFil: Bizzetti Sona, A. M.. Università di Padova; Italia. Istituto Nazionale Di Fisica Nucleare.; ItaliaFil: Della Vedova, F.. Università di Padova; Italia. Istituto Nazionale Di Fisica Nucleare.; ItaliaFil: Grebosz, J.. Polish Academy of Sciences; ArgentinaFil: Haas, F.. Université de Strasbourg; Francia. Centre National de la Recherche Scientifique; FranciaFil: Kmiecik, M.. Polish Academy of Sciences; ArgentinaFil: Maj, A.. Polish Academy of Sciences; ArgentinaFil: Męczyński, W.. Polish Academy of Sciences; ArgentinaFil: Napoli, D. R.. Istituto Nazionale Di Fisica Nucleare.; ItaliaFil: Nespolo, M.. Università di Padova; Italia. Istituto Nazionale Di Fisica Nucleare.; ItaliaFil: Papka, P.. Université de Strasbourg; Francia. Centre National de la Recherche Scientifique; FranciaFil: Sánchez i Zafra, A.. Université de Strasbourg; Francia. Centre National de la Recherche Scientifique; FranciaFil: Styczen, J.. Polish Academy of Sciences; ArgentinaFil: Thummerer, S.. Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung; AlemaniaFil: Ziębliński, M.. Polish Academy of Sciences; Argentin

Transition probabilities in the X(5) candidate 122^{122}Ba

To investigate the possible X(5) character of 122Ba, suggested by the ground state band energy pattern, the lifetimes of the lowest yrast states of 122Ba have been measured, via the Recoil Distance Doppler-Shift method. The relevant levels have been populated by using the 108Cd(16O,2n)122Ba and the 112Sn(13C,3n)122Ba reactions. The B(E2) values deduced in the present work are compared to the predictions of the X(5) model and to calculations performed in the framework of the IBA-1 and IBA-2 models

Drag and Solar Sail Deorbiting: Re-Entry Time Versus Cumulative Collision Probability

Solar and drag sailing have been proposed as passive end-of-life deorbiting methods, and technological demonstrators are under development. For orbit above 800 km altitude solar radiation pressure can be exploited for increasing the orbit eccentricity until the perigee enters the drag region until final re-entry. The performance of the sailing strategy is determined by four parameters: the required effective area-to-mass ratio to deorbit the spacecraft, which determine the sail size given the satellite’s mass, the time to deorbit and the augmented collision probability caused on and by the sail through its passage in the Low Earth Orbit protected region densely populated by space debris. In this paper we assess the sail dimension with respect to the augmented collision risk depending on the sail area and the deorbiting time

g factors of coexisting isomeric states in Pb-188

The $g$ factors of the ${12}^{+}$, {11}^{\ensuremath{-}}, and {8}^{\ensuremath{-}} isomeric states in $^{188}\mathrm{Pb}$ were measured using the time-differential perturbed angular distribution method as g({12}^{+})=\ensuremath{-}0.179(6), g({11}^{\ensuremath{-}})=+1.03(3), and g({8}^{\ensuremath{-}})=\ensuremath{-}0.037(7). The $g$ factor of the ${12}^{+}$ state follows the observed slight down-sloping evolution of the $g$ factors of the ${i}_{13/2}^{2}$ neutron spherical states with decreasing $N$. The $g$ factors of the {11}^{\ensuremath{-}} and {8}^{\ensuremath{-}} isomers proposed as oblate and prolate deformed states, respectively, were interpreted within the rotational model, using calculated and empirical $g$ factor values for the involved single-particle orbitals