1,721,036 research outputs found
Compact supervisors for general constraint enforcement in Petri net models with uncontrollable transitions
No full textPetri net (PN) models of production processes are subject to a number of heterogeneous constraints, both static (e.g., limitation and sharing of resources, job limitation) and behavioral (e.g., liveness, reversibility). All of these constraints can be indirectly formulated as generalized mutual exclusion constraints (GMECs), which are conveniently implemented as monitor places suitably connected to the transitions of the open-loop PN plant model. The design procedure is typically sequential, dealing separately with each control objective, possibly resulting in a redundant supervisor. The process is further complicated in the presence of uncontrollable transitions. An integrated modeling approach is here proposed to solve the redundancy problem by accounting for all the specifications in a single design step that optimizes the number of required GMECs and the permissivity of the resulting supervisor. The supervisor can be then implemented as a monitor-based supervisor or, in some cases, as a logical predicate
Integrated design of optimal supervisors for the enforcement of static and behavioral specifications in Petri net models
No full textPetri net (PN) supervisory control is often performed through a sequential procedure that introduces additional constraint layers over an initial unconstrained PN model, using generalized mutual exclusion constraints (GMECs) implemented as monitor places. This is typical, e.g., in the context of flexible manufacturing systems, where the initial model represents the production sequences and the constraints are used to express static specifications, such as job limitations or the usage of resources, and behavioral ones, as liveness, controllability, etc. This sequential procedure may yield a redundant model, that is not easily reduced a posteriori. Also, it is difficult to ensure maximal permissivity with respect to multiple behavioral specifications. This paper, building on recent results regarding optimal supervisor design with branch & bound methods, proposes an integrated modeling approach that can be used to derive a minimal supervisor guaranteeing the attainment of an arbitrary set of static and behavioral specifications in a maximally permissive way. Among behavioral specifications, deadlock-freeness, liveness, reversibility and behavioral controllability are considered in the paper. The supervisor comes in the form of a simple set of GMECs or of a disjunction of sets of GMECs
Parsimonious Monitor Control of Petri Net models of FMS
No full textMost approaches for deadlock prevention and liveness enforcement in Petri nets rely on siphon control methods or the theory of regions to derive monitor-based supervisors. These techniques raise methodological and computational issues, from the existence of feasible solutions to the hardness of guaranteeing maximal permissivity and optimality in the size and cost of the control subnet. Recently, the supervisor design problem has also been reformulated as a direct monitor optimization task based on integer linear programming, which can more effectively deal with the mentioned issues and objectives. This paper introduces an efficient branch-and-bound scheme for the exploration of the solution space of the direct monitor optimization problem. An extensive computational analysis on a set of benchmark instances demonstrates the efficiency of the approach
A reachability graph partitioning technique for the analysis of deadlock prevention methods in bounded Petri nets
No full textSiphon control is a widespread methodology for deadlock prevention (DP) in Petri net (PN) models. Besides achieving liveness or DP, control methods should also be evaluated regarding their permissivity (in terms of the number of allowed states) and constraint redundancy. This work introduces a partitioning of the reachability graph based on strongly connected components that nicely and compactly illustrates the PN’s evolution behavior, especially regarding liveness, deadlocks and siphon-related properties. The resulting reduced graph is used as a tool for the analysis of DP methods in bounded PNs, to reveal the use of non-maximally permissive constraints and constraint overlapping
Monitor Optimization in Petri Net Control
No full textDeadlock Prevention and Liveness Enforcement in Petri nets are mostly addressed with monitor-based methods, designed indirectly using the concept of siphon or the theory of regions. The obtained control solutions are sometimes maximally permissive, but generally not optimal in terms of the number of monitors, nor in the cost of the control subnet. This paper proposes an optimization approach aiming at the direct design of the monitors, that allows for an easy formalization of the various control objectives. An efficient solution scheme, based on a branch-and-bound approach is also presented. The proposed monitor design algorithm yields significant improvements over known benchmark problems
Parsimonious deadlock-free Petri net models of flexible manufacturing systems
No full textPetri net (PN) modeling of flexible manufacturing systems is generally performed through a sequential procedure that introduces additional constraint layers over an unconstrained PN modeling the production sequences. The constraints account for job limitations, resources usage, liveness, etc., and are typically all representable as generalized mutual exclusion constraints, that can be implemented as monitor places suitably connected to the PN’s transitions. This sequential procedure typically yields a redundant model, that is not easily reduced a posteriori. An integrated modeling approach is here proposed to solve the redundancy problem by enforcing all the constraints in a single design step that optimizes the number of monitors. The monitor design task is reformulated as a classification problem, which is solved through an efficient branch & bound approach. Some examples emphasize the potential model size reductions that can be achieved
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Combined Siphon and Marking Generation for Deadlock Prevention in Petri Nets
No full textIn Petri-net (PN) modeling of flexible manufacturing systems, deadlock prevention is often addressed by means of siphon-control methods. Constraints that avoid the emptying of siphons can be easily implemented using additional places suitably connected to the PN transitions. Efficient siphon-based techniques achieve highly permissive solutions using as few control places as possible. One such technique employs a set-covering approach to optimally match emptiable siphons to critical markings. In this paper, a modified version of the method is proposed that achieves the same results in terms of permissivity and size of the control subnet but avoids full siphon enumeration. This greatly reduces the overall computational time and memory requirements and allows the applicability of the method to large-size models
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