Bruno Maione

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.

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.

Evolutionary adaptation of dispatching agents in heterarchical manufacturing systems

We propose a new approach to job flow adaptive operational control in advanced manufacturing systems. The major feature of the method is the distribution of the control tasks among completely autonomous intelligent agents. Namely, agents are implicitly coordinated by a nature-analogous adaptation mechanism, which continuously tunes the free parameters of the control law of each agent. The proposed approach is effective and reactive to severe disturbances and changes in the manufacturing environment. Simulation experiments illustrate the operational distributed approach and its response to faults.

Controllability of linear single-input positive discrete-time systems

Linear positive systems are utilized in various scientific areas such as economic modelling, behavioural science, stochastic processes etc. A recent study has focused on some sufficient conditions for the controllability of these systems. A full characterization is provided of the controllability for discrete-time single-input positive systems. Necessary and sufficient conditions are derived using a digraph-theoretic approach. The main results show that strong constraints are to be imposed on the digraph associated with the pair (A, b) to ensure the controllability of the system.

Genetic multi-criteria approach to flexible line scheduling

Abstract Job scheduling in flexible production systems is a complex task even for simple cases. This paper deals with this problem using fuzzy set theory and genetic algorithms. Fuzzy techniques allow us to define global performance measures expressing different and often conflicting objectives of the production. Moreover, according to a fuzzy multi-criteria algorithm, we propose a methodology to combine various heuristics, with different weights, in a single dispatching criterion. A genetic optimization process selects the weights guaranteeing good system performances. A case study and some extensive simulations show the efficiency of the methodology.

A control-oriented model of a Common Rail injection system for diesel engines

This paper presents a model of a Common Rail injection system for diesel engines. The model is derived by considering the components of the system as control volumes and applying elementary fluid dynamics and mechanics laws. Suitable simplifications are introduced, to make the model adequate for control purposes, trading off between computational effort and accuracy. The model obtained is a fifth order nonlinear one, in state-space representation, and relies on simple, well defined, geometric parameters of the system. The results obtained are compared with data collected on an experimental setup, and with those obtained through the fluid dynamic simulation AMESIMreg

Heuristic scheduling of jobs on a multi-product batch processing machine

In this paper we study a scheduling problem on a batch processing machine, i.e. on a processor that can manufacture a set of jobs simultaneously. The scheduling objective is to determine the batches of jobs of different types and their loading sequence to fully utilize the processor and to minimize the tool replacing and setting up times. The schedule is subject to some physical and technological constraints due both to the machine capacity and to the limited number of tools at its disposal. The attention given to this problem is motivated by a recurrent scheduling problem occurring in the production management of a typical shoe factory, which is considered as a case study. The paper gives an efficient heuristic procedure leading to a solution close to the optimal one.

Event control for deadlock avoidance in assembly systems

We deal with deadlock problems in automated assembly systems, described by discrete event system models. We synthesize a supervisory controller capable of inhibiting events that could lead the system to a deadlock condition. To characterize the control actions we utilize digraphs describing the interaction between jobs and resources effectively.

Performance of deadlock avoidance algorithms in flexible manufacturing systems

Deadlock is a highly unfavorable situation that can occur in flexible manufacturing systems. The occurrence of a deadlock can cripple parts of a production system and hinder flexible automation. It is necessary to develop control policies that avoid deadlocks by restricting the freedom in resource allocation; however, such policies can also negatively affect system performance indices. Feedback algorithms, which use information on the current operating condition of the system to avoid deadlock, can be distinguished based on the effects of the constraints they impose on the freedom in resource allocation. Referring to this characterization of the algorithms, this paper compares performances of some deadlock avoidance policies. First, theoretical analysis is carried out by introducing a criterion for ordering different policies by flexibility in resource allocation. Second, avoidance policies are applied to some case studies, which are simulated in details. The simulation analysis confirms the theoretical results showing that the algorithms allowing larger flexibility in resource allocation lead to better performance indices.