Automatic tuning of decentralized PID controllers for MIMO processes

An automatic tuning algorithm for decentralized PID control in multiple-input multiple-output (MIMO) plants is presented. This algorithm generalizes the authors' recent auto-tuner for two-input two-output systems to any number of inputs and outputs. The algorithm consists of two stages. In the first, the desired critical point, which consists of the critical gains of all the loops and a critical frequency, is identified. The auto-tuner identifies the desired critical point with almost no a priori information about the process. During the identification phase all controllers are replaced by relays, thus generating limit cycles with the same period in all loops. It is shown that each limit cycle corresponds to a single critical point of the process. By varying the relays parameters different points can be determined. The auto-tuner contains a procedure which converges rapidly to the desired critical point while maintaining the amplitudes of the process variables as well as of the manipulated variables within prespecified ranges. In the second stage, the data of the desired critical point is used to tune the PID controllers by the Ziegler-Nichols rules or their modifications. This paper focuses on the first stage. The steady-state process gains, which are required for the appropriate choice of the desired critical point, are determined by the auto-tuner in closed-loop fashion simultaneously with the identification of the critical point. The identification of the process gains is achieved at no extra plant time. Based upon a large number of simulated cases, the proposed auto-tuner seems to be efficient and robust. The paper discusses the underlying principles of the auto-tuner and its properties and capabilities are demonstrated via examples.     

Technologies for ballast water treatment: a review

Invasive aquatic species discharged through ballast water is one of the most serious problems posed nowadays in the marine environment. This review paper summarizes the available technologies applied for ballast water treatment. These technologies can be either port‐based or ship‐based, with the latter being easier to implement. Special emphasis was given to onboard treatment methods, which can be categorized as physical separation, mechanical or chemical methods. The efficiency of the methods, as well as the capacity of application and the target microorganisms were compiled and are presented in this review. Copyright © 2009 Society of Chemical Industry     

The effect of polyethylene crystallinity and polarity on thermal stability and controlled release of essential oils in antimicrobial films

Antimicrobial packaging can preserve and increase shelf life of free preservatives food products. Active materials present in the packaging material can migrate, in a controlled manner, to the food surface, avoiding bacterial and fungal proliferation and keeping the food product edible for longer periods of time. Essential oils (EO) are natural antimicrobial agents that can be released to the headspace with no direct contact between the package and the food. To minimize loses of EO during high heat melt processing, a three stages process was implemented and tested. Antimicrobial films were prepared by melt mixing a variety of polyethylene copolymers in the presence of organo‐modified montmorillonite nano clay (NC) and thymol, an EO present in oregano and thyme. A controlled EO desorption from films can be achieved by changing the polymer crystallinity and polarity. As the crystallinity increased, the thermal stability of the EO during the extrusion process improved. The addition of NC affects the structure and homogeneity of the crystals. The combination of high polymer crystallinity and chemical affinity between EO and NC increased the thermal stability of the EO during film processing, enabling to control the desorption rate. The effect of multilayer structure based on varied densities and polarities was also studied. Increasing the polarity of the outer layers in multilayered film reduced the EO desorption rate as a result of chemical interactions between the polymer and the EO. The final antimicrobial activity of the films was also found to be dependent on the EO partitioning. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40309.     

Exact and efficient construction of Minkowski sums of convex polyhedra with applications

We present an exact implementation of an efficient algorithm that computes Minkowski sums of convex polyhedra in R³. Our implementation is complete in the sense that it does not assume general position. Namely, it can handle degenerate input, and it produces exact results. We also present applications of the Minkowski-sum computation to answer collision and proximity queries about the relative placement of two convex polyhedra in R³. The algorithms use a dual representation of convex polyhedra, and their implementation is mainly based on the Arrangement package of Cgal, the Computational Geometry Algorithm Library. We compare our Minkowski-sum construction with the only three other methods that produce exact results we are aware of. One is a simple approach that computes the convex hull of the pairwise sums of vertices of two convex polyhedra. The second is based on Nef polyhedra embedded on the sphere, and the third is an output-sensitive approach based on linear programming. Our method is significantly faster. The results of experimentation with a broad family of convex polyhedra are reported. The relevant programs, source code, data sets, and documentation are available at http://www.cs.tau.ac.il/~efif/CD and a short movie [Fogel E, Halperin D. Video: Exact Minkowski sums of convex polyhedra. In: Proceedings of 21st annual ACM symposium on computational geometry. 2005. p. 382-3] that describes some of the concepts portrayed in this paper can be downloaded from http://www.cs.tau.ac.il/~efif/CD/Mink3d.avi.     

Modeling users on the World Wide Web based on cognitive factors,navigation behavior and clustering techniques

This paper focuses on modeling users’ cognitive styles based on a set of Web usage mining techniques on user navigation patterns and clickstream data. Main aim is to investigate whether specific clustering techniques can group users of particular cognitive style using measures obtained from psychometric tests and content navigation behavior. Three navigation metrics are proposed and utilized to find identifiable groups of users that have similar navigation patterns in relation to their cognitive style. The proposed work has been evaluated with two user studies which entail a psychometric-based survey for extracting the users’ cognitive styles, combined with a real usage scenario of users navigating in a controlled Web 2.0 environment. A total of 106 participants of age between 17 and 25 participated in the study providing interesting insights with respect to cognitive styles and navigation behavior of users. Studies like the reported one can be useful for modeling users and assist adaptive Web 2.0 environments to organize and present information and functionalities in an adaptive format to diverse user groups.     

A Low Temperature Route toward Hierarchically Structured Titania Films for Thin Hybrid Solar Cells

The requirement of high‐temperature calcination for titanium dioxide in (solid‐state) dye‐sensitized solar cells (DSSCs) implies challenges with respect to reduced energy consumption and the potential for flexible photovoltaic devices. Moreover, the use of dye molecules increases production costs and leads to problems related with dye bleaching. Therefore, fabrication of dye‐free hybrid solar cells at low temperature is a promising alternative for current DSSC technology. In this work the authors fabricate hierarchically structured titania thin films by combining a polystyrene‐block‐polyethylene oxide template assisted sol–gel synthesis with nano‐imprint lithography at low temperatures. The achieved films are filled with poly(3‐hexylthiophene) to form the active layer of hybrid solar cells. The surface morphology is probed via scanning electron microscopy and atomic force microscopy, and the bulk film morphology is examined with grazing incidence X‐ray scattering. Good light absorption by the active layer is proven by UV–vis spectroscopy. An enhancement in light absorption is observed and ascribed to light scattering in mesoporous titania films with imprinted superstructures. Accordingly a better photovoltaic performance is found for nano‐imprinted solar cells at various angles of light incidence.     

Fast randomized algorithm for 2-hops clustering in vehicular ad-hoc networks

Vehicular Ad-Hoc Networks (VANETs) enable inter vehicle wireless communication as well as communication with road side equipment. Warning messages can be exchanged among nearby vehicles, helping to predict dangerous situations, and thus improving road safety. Such safety messages require fast delivery and minimal delay to local areas, in order for them to be effective. Therefore, a fast and efficient channel access scheme is required. A feasible solution, derived from the Mobile Ad-Hoc Networks (MANETs) field, groups nodes into smaller manageable sections called clusters. Such an approach can be beneficial for locally delivering messages under strict time constraints. In this paper, a Hierarchical Clustering Algorithm (HCA) is presented. HCA is a distributed randomized algorithm, which manages channel access by forming three hierarchy clusters. The proposed channel access scheme enables delay bounded reliable communication. Unlike other common clustering algorithm for VANETs, HCA does not require the knowledge of the vehicles’ locations. This feature guarantees accurate operation even when localization systems such as GPS are not available. The running time and message complexity were analyzed and simulated. Simulation results show that the algorithm behaves well especially under realistic mobility patterns; therefore, it is a suitable solution for channel access scheme for VANETs.     

Geometric Frustration in Molecular Crystals

Unlike Lego bricks that perfectly assemble next to one another, solid assemblies of organic compounds often include some inevitable misfit between constituents, giving rise to geometric frustration. In order to fit into the assembly the molecular building blocks must distort, at some finite energetic cost. In cases where this distortion at the ground state is uniform across all the units in the assembly, the associated geometric frustration is said to be locally resolved. Such locally resolved frustration carries little implications on the morphology and response properties of the assembled structure. However, in many cases, for small enough assemblies there are non-local compromises that are more energetically favorable. These conformations are associated with non-uniform distortions and highly cooperative response between the molecular constituents. The cooperative nature of frustrated assemblies may result in growth arrest, tendency to form filaments, exotic response properties and large morphological variations during the growth of the assembly. Almost a century ago German mineralogist Ferdinand Bernauer discovered that a large fraction of small organic compounds could form twisted molecular crystals. These are straight and narrow needle-like structures with mesoscopic pitch, a crystalographically impossible structure. Recent revived interest in twisted molecular crystals discovered even more compounds that form these exotic assemblies and led to their study by modern means. Electron microscopy revealed straight faceted structures with sharp diffraction peaks in selected area electron diffraction, much like regular crystals. Moreover, the pitch of the molecular crystals varied with size, with thicker crystals exhibiting less twist. In this work we review twisted molecular crystals as frustrated assemblies. In this approach twist emerges from the preferred morphology at the constituent scale, and gets attenuated with size by the incompatibility of twist and large-scale crystalline order. We discuss two distinct mechanisms that produce twisted molecular crystals, and provide a prediction for the twist decay as a function of the crystals’ spatial dimensions.     

Homogeneous Desulfurization of Thiols and Sulfides by Yb[N(SiMe3)2]2.2 THF. Relevance to Heterogeneous and Biological Desulfurizations and the Synthesis of Lanthanide(II) Sulfides

Yb[N(SiMe₃)₂]₂·2THF was found to desulfurize thiols (RSH) and dibenzyl sulfide. With thiols, the extent of desulfurization was a function of the relative stability of the corresponding radical, R·. Radicals were also involved in the desulfurization of dibenzyl sulfide. A unified mechanism involving a series of one-electron redox steps leading to the generation of organic radical intermediates has been proposed.     

The end of cheap oil: Current status and prospects

This paper, analyses the factors impacting the price of crude oil in order to examine the likely evolution of the oil market and attempts to answer the question, whether cheap oil is already a thing of the past. Based on data made publicly available mostly by the major oil companies, it examines the effects of demand and supply, the evolution of world oil reserves, the trends in new discoveries of new oilfields, the evolution of world Gross Domestic Product (GDP), and the depletion of oil resources. The analysis concludes that the world peak in conventional oil production—the point beyond which oil production will irreversibly start declining—is approaching and will be reached, even according to the most optimistic scenarios, before 2040 and quite possibly much sooner. If the appropriate solutions for substituting crude oil and for conserving the use of energy are not implemented in time, then the current upward trend in oil prices is bound to continue.