1,721,590 research outputs found
Continuous problem solving and computational suspiciousness
No full textContinuous optimization seems to be the ubiquitous formulation of an impressive number of different problems in science and engineering. In this chapter, a unified framework for problem solving is proposed in the continuum setting which is based on the notion of action, a sort of continuous algorithm running on an abstract machine, referred to as the deterministic terminal attractor machine (DTAM), somehow related to discrete computational counterparts.
A number of examples are given which illustrate how continuous algorithms can be devised. The proposed general computational scheme incorporates most interesting supervised and unsupervised learning schemes in artificial neural networks as well as the problem solving approach based on Hopfield networks. Finally, a general discussion on computational complexity issues indicates some intriguing links between the presence of local minima in the error surface of the energy function and the complexity of the solution
Characterization and treatment options of residues from waste-to-energy plants
Full text linkLe bottom ash (BA) rappresentano il residuo solido prodotto in maggior quantità dagli impianti di trattamento termico dei rifiuti. Molti paesi europei consentono il riutilizzo di questi materiali nell’industria dei materiali da costruzione, in quella siderurgica e nel settore di sottofondi stradali.
A causa del rischio legato al possibile rilascio di specie contaminanti, le scorie possono porre problemi una volta reimmesse nell’ambiente. Tali materiali, pertanto, prima di essere posti in discarica o proposti per un eventuale recupero, possono necessitare di un idoneo pretrattamento. I trattamenti più semplici e più comunemente utilizzati sono la separazione dimensionale, la rimozione dei metalli magnetici e non magnetici e la maturazione. Fra i trattamenti più avanzati ci sono, invece, i processi di stabilizzazione/solidificazione e quelli termici di vetrificazione, fusione e sinterizzazione. La scelta del trattamento dipende da condizioni sito-specifiche, dagli obiettivi di recupero e dai limiti normativi. In ogni caso, nella scelta del pre-trattamento è necessario porre particolare attenzione ai costi sia operativi che di investimento.
Il principale obiettivo della presente tesi di dottorato è stato quello di valutare l’effetto dei pre-trattamenti di separazione dimensionale e maturazione (o weathering) applicati a residui da processi di incenerimento di rifiuti. In particolare sono stati considerati i seguenti materiali:
- BA da impianti di incenerimento a griglia di RSU;
- BA da impianti di incenerimento a griglia di biomasse;
- Residui vetrificati da impianti di gassificazione ad alta temperatura di combustibile da rifiuti (RDF).
Il trattamento di separazione dimensionale è stato applicato a due campioni di BA provenienti da due diversi impianti di gassificazione di RDF ad alta temperatura. I residui sono stati caratterizzati con riferimento ai seguenti parametri: distribuzione granulometrica, peso specifico dei costituenti solidi, caratteristiche di compattazione, permeabilità, contenuto d’acqua e composizione chimica. Il comportamento a lisciviazione è stato valutato mediante il test di cessione EN 12457-2, il test per la determinazione della Capacità di Neutralizzazione Acida (ANC) secondo la metodica TS 14429 ed il test di Availability (NEN 7371). I risultati della indagini analitiche hanno evidenziato delle marcate differenze fra i due materiali considerati. In un caso le concentrazioni dei metalli pesanti negli eluati sono sempre risultate inferiori rispetto ai limiti imposti dalla normativa per il recupero o il conferimento in discarica per rifiuti inerti ed i contaminanti sono risultati uniformemente distribuiti fra le differenti classi granulometriche. Per l’altro materiale analizzato, le concentrazioni di Cu, Cr e Ni sono risultate superiori rispetto ai limiti fissati per il recupero. Tuttavia, per lo stesso campione, i risultati sperimentali hanno evidenziato una maggiore concentrazione di Cu e Ni nelle frazioni più fini. Di conseguenza il pre-trattamento di separazione dimensionale potrebbe dimostrarsi efficiente nel ridurre il contenuto di tali metalli nel residuo. Dall’altra parte, invece, il Cr è risultato presente in maniera pressoché uniforme in tutte le classi granulometriche, rendendo quindi la separazione dimensionale non idonea per la sua rimozione.
Per quanto riguarda il trattamento di weathering di BA da impianti di incenerimento di RSU e biomasse, i residui sono stati analizzati a successivi steps di trattamento e caratterizzati in termini di composizione chimica e mineralogica, stabilità volumetrica e tendenza a lisciviazione sia a breve che a lungo termine. I metalli risultati più critici sono stati Pb, Zn e Cu. Dopo 12 settimane di trattamento, la concentrazione di Zn è diminuita in maniera significativa per tutti i materiali considerati, mentre per il Pb si è osservata una lieve diminuzione solo per le BA da biomasse. Le concentrazioni di Pb negli eluati si sono rivelate indipendenti dal pH ed inferiori rispetto alle concentrazioni di saturazione fissate da idrossido di piombo (Pb(OH)2), anglesite (PbSO4) e currusite (PbCO3). Il comportamento a lisciviazione del Cu è risultato strettamente correlato a fenomeni di complessazione con il carbonio organico disciolto.
Il test ANC ha permesso di condurre una analisi maggiormente dettagliata del processo di weathering, con particolare riferimento alla correlazione fra trasformazioni di carattere mineralogico e comportamento a lisciviazione dei metalli pesanti.
Il trattamento, infine, si è dimostrato efficace nel garantire ai materiali una adeguata stabilità volumetrica.
Per quanto riguarda l’effetto del weathering applicato a residui da processi di gassificazione ad alta temperatura di RDF, la ricerca è stata condotta su un singolo campione, facendo riferimento alle diverse classi granulometriche. Il trattamento si è dimostrato efficace nel ridurre la tendenza a lisciviazione dei metalli anfoteri quali Zn e Pb, mentre non ha avuto effetti benefici sul comportamento dei metalli che tendono a formare ossianioni, fra cui il Cr. La tendenza a lisciviazione del Cu non è apparsa influenzata dal processo di maturazione. Alla fine del trattamento Cu e Cr sono risultati i metalli più critici per il recupero o il conferimento in discarica per inerti del materiale oggetto di studio.Bottom ash (BA) is the residue produced in greatest amount from incineration of municipal solid waste (MSW). Several countries allow the use of BA for civil engineering applications, as road construction, embankment, pavement, aggregate, filler for concrete, asphalt, or low-cost tiles. In these cases, leaching of heavy metals is a major issue and can limit the feasibility of these reuse options.
In order to comply with limits set by law for leaching of heavy metals, several options are available for treating MSW combustion residues: solidification, stabilization, vitrification, classification by granulometric size particles and ageing or weathering. The choice of the treatment depends on site specific conditions, utilization or disposal objectives and law requirements. When an ash treatment method must be chosen, much attention must be payed to investment and operating costs.
The main objective of this doctoral thesis was to investigate the effect of size separation and weathering as pre-treatments applied to different residues from waste incineration plants. The following materials were chosen for the experimental activities:
- BA from MSW grate furnace combustion plants;
- BA from wood waste grate furnace combustion plants;
- vitrified residues from refuse derived fuel (RDF) high-temperature gasification plants.
Size separation was applied to BA from two high temperature RDF gasification plants. These residues were analysed for the following parameters: grain size distribution, specific gravity, compaction behaviour, hydraulic conductivity, water content and chemical composition. Besides, leaching characteristics were also investigated by means of the EN 12457-2 batch leaching test, the TS 14429 Acid Neutralization Capacity (ANC) test and the Availability Test (NEN 7371). Results of the analytical investigations showed marked differences between the two investigated BA. In one case the levels of heavy metals were always below the Italian regulatory limits for either reuse or disposal as inert waste and a there was a uniform contaminant distribution above the different grain size classes. In the other case Cu, Cr and Ni were above the limits set for reuse. Moreover, the finest fractions appeared to be more contaminated in Cu and Ni and a size separation pre-treatment could be considered as a viable solution to reduce their content in BA. On the other hand, Cr was more or less evenly distributed in all the grain size classes, making the size separation pre-treatment not effective for this metal.
As regards weathering treatment of BA from MSW and wood waste grate furnace combustion plants, the residues were analysed at different steps of treatment and characterized for their chemical and mineralogical composition, volume stability and both short term as well as long term leaching behaviour. Pb, Zn and Cu were the heavy metals to be released in greatest amount. After 12 weeks of treatment the concentration of leached Zn fell significantly in both cases, while for Pb it was possible to observe a concentration decrease only for biomass. Lead concentrations seemed to be independent from the BA pH and undersaturated with respect to lead hydroxide (Pb(OH)2), anglesite (PbSO4) and cerrusite (PbCO3). Leaching of Cu was well described by complexation processes with dissolved organic carbon.
ANC test permitted to carry out analysis of the weathering process, with particular reference to mineralogical transformation and behaviour of heavy metals.
The treatment resulted to be effective to comply with law limits concerning volume stability.
As regards the effects of natural weathering on RDF high-temperature gasification residues, the study was conducted on a single BA sample and with reference to different grain size fractions. Significant results were obtained for the leaching of amphoteric elements such as Zn and Pb, while for oxyanion forming elements, such as Cr, it did not exert beneficial effects. Leaching of Cu appeared to be not affected by weathering. Cr and Cu resulted to be the most critical elements in view of recycling or disposal as inert waste
Optimal Algorithms for Well-Conditioned Nonlinear Systems of Equations
No full textWe propose solving nonlinear systems of equations by function optimization and we give an optimal algorithm which relies on a special canonical form of gradient descent. The algorithm can be applied under certain assumptions on the function to be optimized, that is, an upper bound must exist for the norm of the Hessian, whereas the norm of the gradient must be lower bounded. Due to its intrinsic structure, the algorithm looks particularly appealing for a parallel implementation. As a particular case, more specific results are given for linear systems. Moreover, related results hold also for systems of quadratic equations for which an estimation for the requested bounds can be devised. Finally, we report numerical results in order to establish the actual computational burden of the proposed method and to assess its performances with respect to classical algorithms for solving linear and quadratic equations
An algebraic approach to Cichelli's perfect hashing
No full textThe aim of this paper is to describe a new approach to building minimal and perfect hash functions for a predefined set of keys. Several papers have dealt with this problem and proposed various kinds of functions. This study is based on a function whose address depends both on the letter codes and the letter position in the key, and therefore represents an extension of Cichelli's function. The weights associated with the position are considered to be fixed, and letter code computing is considered to be an interpolation problem. As a result, hash building only requires the solution of an algebraic linear system and then the time complexity is polynomial O(n3). © 1989 BIT Foundations
Active movement restores veridical event-timing after tactile adaptation
No full textTomassini A, Gori M, Burr D, Sandini G, Morrone MC. Active movement restores veridical event-timing after tactile adaptation. J Neurophysiol 108: 2092-2100, 2012. First published July 25, 2012; doi: 10.1152/jn.00238.2012.-Growing evidence suggests that time in the subsecond range is tightly linked to sensory processing. Event-time can be distorted by sensory adaptation, and many temporal illusions can accompany action execution. In this study, we show that adaptation to tactile motion causes a strong contraction of the apparent duration of tactile stimuli. However, when subjects make a voluntary motor act before judging the duration, it annuls the adaptation-induced temporal distortion, reestablishing veridical event-time. The movement needs to be performed actively by the subject: passive movement of similar magnitude and dynamics has no effect on adaptation, showing that it is the motor commands themselves, rather than reafferent signals from body movement, which reset the adaptation for tactile duration. No other concomitant perceptual changes were reported (such as apparent speed or enhanced temporal discrimination), ruling out a generalized effect of body movement on somatosensory processing. We suggest that active movement resets timing mechanisms in preparation for the new scenario that the movement will cause, eliminating inappropriate biases in perceived time. Our brain seems to utilize the intention-to-move signals to retune its perceptual machinery appropriately, to prepare to extract new temporal information
Encoding Non-Deterministic Fuzzy Tree Automata Into Recursive Neural Networks
No full textFuzzy neural systems have been a subject of great interest in the last few years, due to their abilities to facilitate the exchange of information between symbolic and subsymbolic domains. However, the models in the literature are not able to deal with structured organization of information, that is typically required by symbolic processing. In many application domains, the patterns are not only structured, but a fuzziness degree is attached to each subsymbolic pattern primitive. The purpose of this paper is to show how recursive neural networks, properly conceived for dealing with structured information, can represent nondeterministic fuzzy frontier-to-root tree automata. Whereas available prior knowledge expressed in terms of fuzzy state transition rules are injected into a recursive network, unknown rules are supposed to be filled in by data-driven learning. We also prove the stability of the encoding algorithm, extending previous results on the injection of fuzzy finite-state dynamics in high-order recurrent networks
Computational capabilities of local-feedback recurrent networks acting as finite-state machines
No full textIn this paper we explore the expressive power of recurrent networks with local feedback connections for symbolic data streams, We rely on the analysis of the maximal set of strings that can be shattered by the concept class associated to these networks (i.e., strings that can be arbitrarily classified as positive or negative), and find that their expressive power is inherently limited, since there are sets of strings that cannot be shattered, regardless of the number of hidden units. Although the analysis holds for networks with hard threshold units, we claim that the incremental computational capabilities gained when using sigmoidal units are severely paid in terms of robustness of the corresponding representation
The Bubble of Web Visibility
No full textThe promotion of visibility as seen through the unique lens of search engines is discussed. Web page visibility was invented by the dragons to protect the treasure and to make it accessible. Following general principles of information retrieval, the documents that match a query are sorted by an index that takes into account their similarity to the query and the absolute authority of the pages themselves. Web page visibility becomes fully defined only at query times, when retrieved pages are sorted for presentation to the user
On the problem of local minima in backpropagation
No full textSupervised learning in multilayered neural networks (MLN's) has been recently proposed through the well-known backpropagation (BP) algorithm. This is a gradient method that can get stuck in local minima, as simple examples can show. In this paper, some conditions on the network architecture and the learning environment, which ensure the convergence of the BP algorithm, are proposed. It is proven in particular that the convergence holds if the classes are linearly separable. In this case, the experience gained in several experiments shows that MLN's exceed perceptrons in generalization to new examples
Variational Laws of Visual Attention for Dynamic Scenes
No full textComputational models of visual attention are at the crossroad of disciplines like cognitive science, computational neuroscience, and computer vision. This paper proposes a model of attentional scanpath that is based on the principle that there are foundational laws that drive the emergence of visual attention. We devise variational laws of the eye-movement that rely on a generalized view of the Least Action Principle in physics. The potential energy captures details as well as peripheral visual features, while the kinetic energy corresponds with the classic interpretation in analytic mechanics. In addition, the Lagrangian contains a brightness invariance term, which characterizes significantly the scanpath trajectories. We obtain differential equations of visual attention as the stationary point of the generalized action, and we propose an algorithm to estimate the model parameters. Finally, we report experimental results to validate the model in tasks of saliency detection
- …
