Maria Pia Fanti

Deadlock control methods in automated manufacturing systems

As more and more producers move to use flexible and agile manufacturing as a way to keep them with a competitive edge, the investigations on deadlock resolution in automated manufacturing have received significant attention for a decade. Deadlock and related blocking phenomena often lead to catastrophic results in automated manufacturing systems. Their efficient handling becomes a necessary condition for a system to gain high productivity. This paper intends to present a tutorial survey of state-of-the art modeling and deadlock control methods for discrete manufacturing systems. It presents the updated results in the areas of deadlock prevention, detection and recovery, and avoidance. It focuses on three modeling methods: digraphs, automata, and Petri nets. Moreover, for each approach, the main and relevant contributions are selected enlightening pros and cons. The paper concludes with the future research needs in this important area in order to bridge the gap between the academic research and industrial needs.

Event-based feedback control for deadlock avoidance in flexible production systems

Modern production facilities (i.e. flexible manufacturing systems) exhibit a high degree of resource sharing, a situation in which deadlocks (circular waits) can arise. Using digraph theoretic concepts we derive necessary and sufficient conditions for a deadlock occurrence and rigorously characterize highly undesirable situations (second level deadlocks), which inevitably evolve to circular waits in the next future. We assume that the system dynamics is described by a discrete event dynamical model, whose state provides the information on the current interactions job-resources. This theoretic material allows us to introduce some control laws (named restriction policies) which use the state knowledge to avoid deadlocks by inhibiting or by enabling some transitions. The restriction policies involve small on-line computation costs, so they are suitable for real-time implementation. For a meaningful class of systems one of these policies is the least restrictive deadlock-free policy one can find, namely it inhibits resource allocation only if leads directly to a deadlock. Finally, the paper discusses the computational complexity of all the proposed restriction policies and shows some examples to compare their performances.

Design and optimization of integrated E-supply chain for agile and environmentally conscious manufacturing

An agile and environmentally conscious manufacturing paradigm refers to the ability to reconfigure a flexible system quickly, economically, and environmentally responsibly. In modern manufacturing enterprises, e-supply chains integrate Internet and web-based electronic market and are promising systems to achieve agility. A key issue in the strategic logistic planning of integrated e-supply chains (IESCs) is the configuration of the partner network. This paper proposes a single- and multiobjective optimization model to configure the network of IESCs. Considering an Internet-based distributed manufacturing system composed of different stages connected by material and information links, a procedure is presented to select the appropriate links. A set of performance indices is associated with the network links. Single-criterion and multicriteria optimization models are presented under structural constraint definitions. The integer linear programming (ILP) problem solution provides different network structures that allow to improve supply chain (SC) flexibility, agility, and environmental performance in the design process. The proposed optimization strategy is applied to two case studies describing two networks for desktop computer production.

On-line fault detection in discrete event systems by Petri nets and integer linear programming ☆

The paper addresses the fault detection problem for discrete event systems in a Petri Net (PN) framework. Assuming that the structure of the PN model and the initial marking are known, faults are modelled by unobservable transitions. Moreover, we assume that there may be additional unobservable transitions associated with the system legal behaviour and that the marking reached after the firing of any transition is unknown. The proposed diagnoser works on-line: it waits for the firing of an observable transition and employs an algorithm based on the definition and solution of some integer linear programming problems to decide whether the system behaviour is normal or exhibits some possible faults. The results characterize the properties that the PN modelling the system fault behaviour has to fulfill in order to reduce the on-line computational effort.

A multi-level approach for network design of integrated supply chains

Integrated e-supply chains are distributed manufacturing systems composed of various resources belonging to different companies and integrated with streamlined material, information and financial flow. The configuration of the supply-chain network is essential for business to pursue a competitive advantage and to meet the market demand. This paper proposes a three-level hierarchical methodology for a supply chain network design at the planning-management level. The integrated supply chain network is described as a set of consecutive stages connected by communication and transportation links, and the configuration aim consists in selecting the actors of the stages on the basis of transportation connection and information flow. More precisely, the first level evaluates the performance of the entities candidate to join the network and singles out efficient elements. The second level solves a multi-criteria integer linear optimization problem to configure the network. Finally, the third level is devoted to evaluating and validating the solution proposed in the first two levels. The overall decision process is the result of the interaction of the modules that are dedicated to each decision level. The paper presents some optimization techniques to synthesize the first two levels and illustrates the hierarchical decision process by way of a case study.

Event-based controller to avoid deadlock and collisions in zone-control AGVS

Automated Guided Vehicle Systems (AGVSs) are the most flexible means to transport pieces among workstations of an Automated Manufacturing System (AMS). The traffic control of such systems must be in charge of avoiding collisions and deadlock conditions. This paper formulates a zone-control scheme to face this problem in real time, based of the knowledge of the AGVS operative conditions. Starting from some theoretical results already stated in the context of deadlock avoidance in AMS, some algorithms are proposed to control, in real time, path assignments to the vehicles and their moves from zone to zone in the system. A final discussion compares the proposed approach with other methods offered by the recent literature.

An urban traffic network model via coloured timed Petri nets

This paper deals with modelling of traffic networks (TNs) for control purposes. A modular framework based on coloured timed Petri nets (CTPNs) is proposed to model the dynamics of signalized TN systems: places represent link cells and crossing sections, tokens are vehicles and token colours represent the routing of the corresponding vehicle. In addition, ordinary timed Petri nets model the signal timing plans of the traffic lights controlling the area. The proposed modelling framework is applied to a real intersection located in Bari, Italy. A discrete event simulation of the controlled intersection validates the model and tests the signal timing plan obtained by an optimization strategy presented in the related literature.

Real time identification of discrete event systems using Petri nets

The paper defines the identification problem for Discrete Event Systems (DES) as the problem of inferring a Petri Net (PN) model using the observation of the events and the available output vectors, that correspond to the markings of the measurable places. Two cases are studied considering different levels of the system knowledge. In the first case the place and transition sets are assumed known. Hence, an integer linear programming problem is defined in order to determine a PN modelling the DES. In the second case the transition and place sets are assumed unknown and only an upper bound of the number of places is given. Hence, the identification problem is solved by an identification algorithm that observes in real time the occurred events and the corresponding output vectors. The integer linear programming problem is defined at each observation so that the PN can be recursively identified. Some results and examples characterize the identified PN systems and show the flexibility and simplicity of the proposed technique. Moreover, an application to the synthesis of supervisory control of PN systems via monitor places is proposed.

Controllability of multi-input positive discrete-time systems

Linear discrete-time positive systems arise in various fields of science, such as economic modelling, behavioural science, stochastic processes, etc. Recently, some authors have examined the problem of the controllability of such systems for the single-input case. This paper provides necessary and sufficient conditions for characterizing the controllability properties of multi-input systems. The results here developed show that only a narrow class of these systems enjoys the properties of reachability and controllability.

Comparing digraph and Petri net approaches to deadlock avoidance in FMS

Flexible manufacturing systems (FMSs) are modern production facilities with easy adaptability to variable production plans and goals. These systems may exhibit deadlock situations occurring when a circular wait arises because each piece in a set requires a resource currently held by another job in the same set. Several authors have proposed different policies to control resource allocation in order to avoid deadlock problems. These approaches are mainly based on some formal models of manufacturing systems, such as Petri nets (PNs), directed graphs, etc. Since they describe various peculiarities of the FMS operation in a modular and systematic way, PNs are the most extensively used tool to model such systems. On the other hand, digraphs are more synthetic than PNs because their vertices are just the system resources. So, digraphs describe the interactions between jobs and resources only, while neglecting other details on the system operation. The aim of this paper is to show the tight connections between the two approaches to the deadlock problem, by proposing a unitary framework that links graph-theoretic and PN models and results. In this context, we establish a direct correspondence between the structural elements of the PN (empty siphons) and those of the digraphs (maximal-weight zero-outdegree strong components) characterizing a deadlock occurrence. The paper also shows that the avoidance policies derived from digraphs can be implemented by controlled PNs.