Currently, conflict-free routing in AGV systems is established by means of one of the following three approaches: (i) the problem elimination through the adoption of a segmented path flow or tandem queue configuration, (ii) the identification of imminent collisions through forward sensing and their aversion through vehicle backtracking and/or rerouting, or (iii) the imposition of zone control and extensive route pre-planning, typically based on deterministic timing of the vehicle traveling and docking stages. Among these three approaches, the segmented path flow-based approach presents the highest robustness to the system stochasticities/randomness, but at the cost of restricted vehicle routings and the need for complicated handling operations. This paper proposes an alternative conflict resolution strategy that will ensure robust AGV conflict resolution, while maintaining the operational flexibility provided by free vehicle travel on arbitrarily structured guidepath networks. Specific...

Deadlock avoidance in sequential resource allocation systems is a well-defined problem in Discrete Event System literature, as it underlies the operation of many contemporary technological systems. In the past, the problem has been studied by means of a number of formal frameworks, including the finite state automata (FSA) and Petri nets (PN). In this paper, it is shown that a significant class of deadlock avoidance policies (DAP), known as algebraic PK-DAP's, originally developed in the FSA paradigm, can be analyzed using recent results from PN structural analysis. Furthermore, the approach to DAP analysis and design taken in this paper has led to the effective generalization of the currently available algebraic PK-DAP's, and to their enrichment with new and more flexible policy implementations. 1 Introduction Deadlock avoidance is an essential control requirement for automated operation of modern technological systems that involve resource sharing. In these environments, th...

Overwhelmed by the complexity of the FMS Deadlock Avoidance problem, current research has, for the most part, ignored the aspects (and benefits) related to flexible (dynamic) job routing. Extending current structural control policies, based on static job routing, to accommodate routing flexibility is nontrivial, primarily due to the fact that the possible routing options for a single job can grow exponentially fast. Hence, computationally efficient techniques are required to incorporate the inherent FMS routing flexibility to current structural control schemes. This paper undertakes the investigation of the problem of integrating routing flexibility in FMS structural control, by addressing the problem of "optimal" job re-routing in case of operational contingencies. Analytical formulations and efficient solution algorithms are developed for the case that the FMS is structurally controlled by a class of recently emerging polynomial-complexity, one-step lookahead deadlock avoid...

Current strategic and technological trends in discrete-part manufacturing require extensive and #exible automation of the underlying production systems. However, even though a great deal of work has been done to facilitate manufacturing automation at the hardware component level, currently there is no adequately developed control methodology for these environments.

Liveness-enforcing supervision of sequential resource allocation systems is currently a well-defined problem, underlying the operation of many contemporary technological systems, spanning a wide spectrum of applications. This technical note provides a brief overview of the currently available results, delineating, both, our major analytical understandings/characterizations concerning the problem concepts/structure and its complexity, and also, our ability to synthesize effective and computationally tractable solutions to it. The last part of the document identifies open / unaddressed research issues, the resolution of which will extent the power of the current theory and will allow the integration of the developed results in the broader control frameworks managing the behavior of these environments.

Although liveness enforcing supervision (LES) of sequential resource allocation systems (RAS) is currently a well-established problem in the Discrete Event System literature, all prior work on it has addressed the underlying LES synthesis problem under the assumption that the system behavior is totally controllable. The work presented in this paper seeks to develop correct and scaleable LES for RAS that present uncontrollability with respect to: 1) the timing of some requested resource allocations, i.e., these allocations will take place as long as the requested resources are available and/or 2) the routing of certain job instances that, after some processing stages, might request special treatment or rework. In addition, the last part of the paper addresses the accommodation in the original LES synthesis problem of externally imposed logical constraints, that constitute "forbidden state" specifications and possess a linear characterization with respect to the system resource allocation state. All problems are addressed in the context of Conjunctive/Disjunctive (CD)-RAS, that constitutes one of the broadest RAS classes investigated in the literature, allowing for arbitrarily structured resource allocations associated with the various process stages, and process routing flexibility.

Traditionally, the semiconductor manufacturing industry has been driven by continuous technological advancement of the underlying production processes. Yet, as the industry matures, mere technology development is no longer sufficient. The effective deployment and exploitation of the system production capacity and operational capability become critical for competitive success. Hence, currently, there is an increasing interest towards the development of a control paradigm / framework that will allow the effective and efficient deployment and operation of contemporary fabs, including the upcoming 300mm fab. The research program presented in this paper seeks to define a detailed modeling and control framework for the real-time 300mm fab operations, by exploiting and integrating emerging results in Discrete Event Systems theory. The proposed approach is demonstrated through a small-scale example, modeling the operation of a 300mm fab bay. Furthermore, in addition to the development of the formal specification, the presented program will also implement the proposed fab modeling and control framework in a Web-based simulation platform, that will function as a fab (re-)configuration and control synthesis tool for the fab control engineer, and as an educational tool for manufacturing system modeling and control.

When a set of processes must share a set of common resources, deadlock problems can arise. To face them, three approaches are usually applied: deadlock detection and recovery, deadlock prevention or deadlock avoidance. In this paper we present one prevention and two avoidance deadlock algorithms for a class of models that appear in manufacturing systems. We use Petri nets as the tool for both, modelling and controlling the system. The prevention algorithm is based on an intensive use of the structure of the Petri net model. The avoidance algorithms correspond to improvements to the well-known Banker's algorithm introduced by Dijkstra in the framework of Operating Systems. 1 Introduction In [1] we proposed a new method for deadlock prevention in Flexible Manufacturing Systems (FMS). To do that, we modelled the manufacturing system by means of a class of Petri nets that we called S 4 PR. One of such nets is composed of a set of state machines (one per type of part to be processed in ...

Traditionally, the semiconductor manufacturing industry has been driven by continuous technological advancement of the underlying production processes. Yet, as the industry matures, mere technology development is no longer sufficient. The effective deployment and exploitation of the system production capacity and operational capability become critical for competitive success. Hence, currently, there is an increasing interest towards the development of a control paradigm / framework that will allow the effective and efficient (re-)deployment and operation of contemporary fabs. The research program presented in this paper seeks to define a detailed modeling and control framework for the real-time fab operations, by exploiting and integrating emerging results in Discrete Event Systems theory. Furthermore, in addition to the development of the formal specification, the presented program will also implement the proposed fab modeling and control framework in a Web-based simulation platform, ...

this paper.