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    A new method for the redundancy analysis of Petri net-based liveness enforcing supervisors
    (Sage Publications Ltd, 2017) Gelen, Gokhan; Uzam, Murat; Li, ZhiWu
    The deadlock control of flexible manufacturing systems (FMSs) has been widely studied in the literature. Petri nets (PNs) are extensively used as a tool for modelling, analysis and controller synthesis of such systems. In general, Petri-net-based liveness enforcing supervisors (LESs) include control places (CPs) together with their input/output arcs. It is well known that the methods proposed for computing CPs may provide redundant and necessary CPs. In this paper, a new method is proposed for redundancy test of CPs by means of supervisory control theory (SCT). The proposed method is based on the idea that after the removal of a CP from an LES, if the controlled model is still live, then the removed CP is redundant. The proposed method makes use of the TCT implementation tool of SCT. It is applicable to a PN-based LES consisting of a set of CPs. The applicability of proposed method is demonstrated by means of examples from the relevant literature. For some examples, the redundancy test provides more permissive behaviour with structurally simpler supervisors.
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    Think-globally-act-locally approach with weighted arcs to the synthesis of a liveness-enforcing supervisor for generalized Petri nets modeling FMSs
    (Elsevier Science Inc, 2016) Uzam, Murat; Gelen, Gökhan; Saleh, Tahir Lawan
    This paper proposes a general approach, called a think-globally-act-locally method with weighted arcs (TGALW), which can be used to compute a liveness-enforcing supervisor (LES) for the Petri net (PN) model of a flexible manufacturing system (FMS) prone to deadlocks. Compared with siphon-based deadlock prevention policies that are usually rather conservative, the method is especially effective for generalized PN classes such as (SR)-R-4 and (SPR)-P-4 and also leads to optimal or near optimal LESS while maintaining the necessary computations simple. An original PNM is first transformed into a net that is behaviorally equivalent to it. A global sink/source place (GP) is used temporarily in the design steps and is finally removed when the liveness of the system is achieved. At each iteration step, bad markings are identified and then controlled by monitors through an established place-invariant based method. A condition of the optimal controllability of a bad marking is proposed, i.e., if all bad markings are optimally controlled at each iteration step, the TGALW method can find an optimal LES. Redundancy of monitors is checked. Typical examples existing in the literature are used to demonstrate applicability and the effectiveness of the proposed method. (C) 2015 Elsevier Inc. All rights reserved.