Dumitru Amarandei

Scheduling availability of discrete event systems

Fault detection is a crucial and challenging task in the automatic control of complex systems, e.g. in flexible manufacturing systems (FMSpsilas) as a representative class of discrete event systems (DESpsilas). In this paper a discrete event system approach to the problem of failure diagnosis is presented. We propose a systematic procedure for detection of failure events using diagnoses implemented with stochastic coloured Petri nets (SCPN). An analytical approach for the availability evaluation of cellular manufacturing systems (as basic components of FMSpsilas) is presented, where a FMS is considered operational as long as its production capacity requirements are satisfied. The approach is used to evaluate transient and steady-state performance of alternative designs based on an industrial example. Possible extensions of the work are also discussed.

Control Charts of Workflows

This paper focuses on the control of the performance characteristics of workflows modeled with stochastic Petri nets (SPNs). This goal is achieved by focusing on a new model for Artificial Social Systems (ASSs) behaviors, and by introducing equivalent transfer functions for SPNs. ASSs exist in practically every multi-agent system, and play a major role in the performance and effectiveness of the agents. This is the reason why we introduce a more suggestive model for ASSs. To model these systems, a class of Petri nets is adopted, and briefly introduced in the paper. This class allows representing the flow of physical resources and control information data of the ASSs components. In the analysis of SPN we use simulations in respect to timing parameters in a generalized semi-Markov process (GSMP). By using existing results on perturbation (e.g., delays in supply with raw materials, derangements of equipments, etc.) analysis and by extending them to new physical interpretations we address unbiased sensitivity estimators correlated with practical solutions in order to attenuate the perturbations.

Intelligent control of dependability workflows

This paper focuses on evaluation of the performance characteristics of workflows in constructions modeled with stochastic Petri nets (SPN). This goal is achieved by focusing on a new model for artificial social systems (ASS) behaviors. ASS exists in practically every multi-agent system, and play a major role in the performance and effectiveness of the agents. This is the reason why we introduce a more suggestive model for ASS. To model these systems, a class of Petri nets is adopted, and briefly introduced in the paper. This class allows representing the flow of physical resources and control information data of the ASSpsilas components. In the analysis of SPN we use simulations in respect to timing parameters in a generalized semi-Markov process (GSMP). By using existing results on perturbation (e.g., delays in supply with raw materials, derangements of equipments, etc.) analysis and by extending them to new physical interpretations we address unbiased sensitivity estimators correlated with practical solutions in order to attenuate the perturbations. An important advantage of this approach is that one simulation is needed for evaluating the stochastic Petri nets and the perturbation analysis and to take advantage of the state of the art.

Method and device for dynamic modelling of rubbery materials applied to human soft tissues. Part I: determination of mechanical characteristics and dynamic model proposal

The paper aims to study the dynamic behaviour human soft tissues, based on the similarity noticed between the tensile stress-strain curves, for soft biological tissues and rubbery materials. Elastic materials, like rubber and steel, were tested for different types of loadings. In order to establish a dynamical model, the force-elongation dependency of the rubber wire is required. A rubber wire was loaded axially by increasing loads and the deformations up to more than 100% strains were registered. The experimental points from force-displacement plot were interpolated and the function proposed was chosen.

Numerical Simulation of Slip-Stick Elastic Contact

Fretting defines a condition in which mechanical contacts are subjected to alternating tangential displacements, small compared to dimensions of contact area, due to oscillating loading conditions. Fretting wear and fretting fatigue are between the most important factors responsible for contact failure, especially when high loads are transmitted through non-conforming contacts, leading to highly localized stress concentrators in the vicinity of the contact region. Prediction of life span of machine elements working in such conditions requires assessment of stress and strain in the contacting bodies, which is the main subject of Contact Mechanics. Although fretting is intrinsically a multidisciplinary process, involving adhesion, oxidation, abrasion and pitting, modern approach suggests that contact stresses play a chief role.

DYNAMIC ASPECTS OF DAMPED ELASTIC IMPACT PART 1. INFLUENCE OF COEFFICINT OF RESTITUTION

The paper presents kinematical and dynamical aspects concerning damped elastic collision between two spheres. The characteristic parameters of system dynamic behaviour are the coefficient of restitution and the initial indentation velocity. The presented results are obtained by integrating a non-linear ordinary differential equation, made in two stages: the first step was an analytical integration followed by a numerical one. The work is parted in two: the first part presents the influence of the coefficient of restitution and the second part is dedicated to the influence of initial indentation velocity upon kinematical and dynamical system parameters.

Formalisms for modelling the discrete manufacturing systems

Considering that in a discrete manufacturing system the production rates of machines are stochastic and not equal, we focus on transient analysis of Markov models. The system is decomposed resulting in reduced computation burden into several base-elements, which determine the event occurrence such as: a machine fails, a machine is repaired, a buffer fills up, a buffer empties. The global properties of machines are determined with the computation of distribution of time to absorption in Markov models of manufacturing systems with deadlocks of failures. It is shown that a discrete-event system formulation and state partition into basic cells and fast and slow varying section, lead to a reduced computation cost.