Reliability modelling of automated guided vehicles using Petri nets

This is an Accepted Manuscript of a book chapter published by Routledge in Risk, Reliability and Safety: Innovating Theory and Practice: Proceedings of ESREL 2016 on September 13 2016, available online: http://www.routledge.com/9781138029972.Automated guided vehicles (AGVs) are being extensively used due to their attributions of high efficiency and low costs. To assure their added value, taking a typical AGV transport system as an example, the reliability issues in AGVs are investigated in this paper. First of all, the AGV transport system was mod-elled as a phased mission that comprises a few key phases. Then, the Petri net (PN) method is applied to describe the logic of the whole phase mission and based on this, the reliability of the mission is assessed via Monte-Carlo simulation. In order to validate the reliability assessment result by the PN method, the theoretical reliability of the AGV system is also assessed through performing fault tree analysis (FTA). The comparison indicates that both methods give very similar results. Thus, it can be concluded that apart from FTA, the PNs method is also a reliable tool for AGV system reliability assessment

Tradable Pollution Permits and the Regulatory Game

This paper analyzes polluters\u27 incentives to move from a traditional command and control (CAC) environmental regulatory regime to a tradable permits (TPP) regime. Existing work in environmental economics does not model how firms contest and bargain over actual regulatory implementation in CAC regimes, and therefore fail to compare TPP regimes with any CAC regime that is actually observed. This paper models CAC environmental regulation as a bargaining game over pollution entitlements. Using a reduced form model of the regulatory contest, it shows that CAC regulatory bargaining likely generates a regulatory status quo under which firms with the highest compliance costs bargain for the smallest pollution reductions, or even no reduction at all. As for a tradable permits regime, it is shown that all firms are better off under such a regime than they would be under an idealized CAC regime that set and enforced a uniform pollution standard, but permit sellers (low compliance cost firms) may actually be better off under a TPP regime with relaxed aggregate pollution levels. Most importantly, because high cost firms (or facilities) are the most weakly regulated in the equilibrium under negotiated or bargained CAC regimes, they may be net losers in a proposed move to a TPP regime. When equilibrium costs under a TPP regime are compared with equilibrium costs under a status quo CAC regime, several otherwise paradoxical aspects of firm attitudes toward TPP type reforms can be explained. In particular, the otherwise paradoxical pattern of allowances awarded under Phase II of the 1990 Clean Air Act\u27s acid rain program, a pattern tending to favor (in Phase II) cleaner, newer generating units, is explained by the fact that under the status quo regime, a kind of bargained CAC, it was the newer cleaner units that were regulated, and which therefore had higher marginal control costs than did the largely unregulated older, plants. As a normative matter, the analysis here implies that the proper baseline for evaluating TPP regimes such as those contained in the Bush Administration\u27s recent Clear Skies initiative is not idealized, but nonexistent CAC regulatory outcomes, but rather the outcomes that have resulted from the bargaining game set up by CAC laws and regulations

Early colonization of stone by freshwater lichens of restored habitats: A case study in northern Italy

This study evaluated the effectiveness and life-strategies of freshwater lichens in colonizing newly constructed stone structures in low-elevation streams in a small nature reserve in northern Italy. Species richness, size of thalli, morphological and ontogenetic traits of the species were related to the age of restored habitats. Lichen colonization was surprisingly rapid, indicating the high potential of these organisms in colonizing restored habitats. However, the species pool found in the restored habitats was different than that found in natural sites in the same study area. The age of newly constructed habitats influenced both species richness and thallus size of the two most frequent Verrucaria species. Verrucaria aquatilis was a rapid colonizer invading the substrate by several small-sized and thin thalli which soon supported a large number of small perithecia whose development began in the earlier phase of thallus formation. V. elaeomelaena, on the contrary, developed according to a different strategy, establishing a thick thallus on which relatively large perithecia were formed much later than in V. aquatilis. As these taxa are important photoautotrophic components of freshwater ecosystems more ecological knowledge is needed to evaluate the effectiveness of different measures of river restoration on lichen communities. The main practical implication of our study is related to the value of small stone structures, such as riffles and ramps, for enhancing the establishment of pioneer freshwater lichens to rapidly colonize newly available substrata. \ua9 2009 Elsevier B.V. All rights reserved

Gallager bounds for space-time codes

Extended summary⎯A quasi-static Rayleigh fading channel is often assumed in space-time coding [1]. Since the decay slope of the pairwise error probability is limited by the diversity order while the number of codewords typically increases exponentially with distances, the standard union bound will diverge even at high signal-to-noise ratios. This is an instance of the weakness of the standard union bound. In another instance for turbo-like codes operating beyond the cutoff rate, this weakness can be overcome by a family of bounding techniques collectively interpreted as “Gallager bounds”. In this paper, we develop efficient and tight bounding techniques for space-time codes in the framework of Gallager bounds. Unlike turbo-like codes where Gaussian noise is the limiting factor, here the Gallager region is defined in the space of fading coefficients. As such the derivation is totally different. Recently, Stefanov and Duman [2] propose a tight bound on the frame error probability of space-time codes in quasi-static fading, which takes the form Pf ≤ ∫ min 1, LPE ( α) fα ( α) dα where α is the nR × nT matrix of fading coefficients with pdf fα ( α) , L is the frame size, and PE ( α) denotes the union bound on the first error event probability for given α. Unfortunately, a multi-dimensional integral is involved. The integration, typically implemented in the Monte Carlo method, is lengthy especially for low error probabilities. In this paper, we circumvent this difficulty by making use of Gallager’s inequality [3, p. 24] Prob(error) ≤ Prob(error, α ∉ R) + Prob ( α∈R where R is the Gallager region where deep fade occurs. Then we have P ≤ L P ( α) f ( α) dα+ f ( α) dα. f E α