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Öğe A hybrid approach to supervisory control of discrete event systems coupling RW supervisors to Petri nets(SPRINGER LONDON LTD, 2006) Uzam, M; Wonham, WMIn this paper a hybrid approach is proposed for supervisory control of discrete event systems (DES) subject to forbidden states. Assuming that an uncontrolled bounded Petri net (PN) model of a (plant) DES and a set of forbidden state specifications are given, the proposed approach computes a maximally permissive and nonblocking closed-loop hybrid model. The first step is to simplify the given PN model by means of PN reduction rules. The simplified model and the specifications are then represented as buffers, and supervisory control theory (SCT) is applied to obtain a Ramadge-Wonham (RW) supervisor in the form of an automaton. After reduction of the latter's state size by a 'control congruence', the simplified RW supervisor is represented by a so-called auto-net and coupled to the given uncontrolled PN plant model by means of inhibitor arcs to represent the disabling actions. The plant model and supervisor auto-net run concurrently, synchronizing on shared events. This procedure provides a maximally permissive and nonblocking 'hybrid' (mixed PN/automaton) closed-loop controlled system. The method is straightforward logically, graphically, and technologically. Its applicability is shown by two examples, one of them a workcell from the PN control literature.Öğe An improved iterative synthesis method for liveness enforcing supervisors of flexible manufacturing systems(TAYLOR & FRANCIS LTD, 2006) Uzam, M; Zhou, MCOur previous work presented a Petri net-based iterative synthesis policy for deadlock prevention in flexible manufacturing systems (FMS). Given the Petri net model of an FMS prone to deadlock, it aims to synthesize a live controlled Petri net. Its use for FMS control guarantees its deadlock-free operation and high performance in terms of resource utilization and system throughput. At each iteration, a first-met bad marking is singled out from the reachability graph of the Petri net. A well-established invariant-based control method is used to prevent it from being reached. This process is carried out until the net model becomes live. The method proposed is generally applicable, easy to use, effective, and straightforward, although its off-line computation is of exponential complexity. This paper presents two improvements: (a) using the Petri net reduction approach to simplify very large Petri net models so as to alleviate computation effort; and (2) simplifying the invariant-based control method. A number of FMS deadlock problems from the literature are used to illustrate them.Öğe An optimal deadlock prevention policy for flexible manufacturing systems using Petri net models with resources and the theory of regions(SPRINGER-VERLAG LONDON LTD, 2002) Uzam, MIn this paper, an optimal deadlock prevention policy for flexible manufacturing systems (FMSs) is proposed. In an FMS, deadlocks can arise because of a limited number of shared resources, i.e. machines, robots, buffers, fixtures etc. Deadlock is a highly undesirable situation, where each of a set of two or more jobs keeps waiting indefinitely for the other jobs in the set to release resources. The proposed optimal deadlock prevention policy is based on the use of reachability graph analysis of a Petri net model (PNM) of a given FMS and the synthesis of a set of not, net elements, namely places with initial marking and related arcs, to be added to the PNM, using the theory of regions, which is a formal synthesis technique to derive Petri nets from automaton-based models. The policy proposed is optimal in the sense that it allows the maximal use of resources in the system according to the production requirements. Two examples are provided for illustration.Öğe Comments on "Feedback control logic for forbidden-state problems of marked graphs: Application to a real manufacturing system"(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2004) Uzam, M; Wonham, WM[Abstract Not Available]Öğe Digital hardware implementation of Petri net based specifications: Direct translation from safe automation Petri nets to circuit elements(TECH UNIV COMP ENGN ELECTR INST, 2001) Uzam, M; Avci, M; Yalcin, MK; Adamski, M; Wegrzn, MIn this paper, a new method is proposed for the digital hardware implementation of Petri net based specifications. The purpose of this paper is to introduce a new discrete event control system paradigm, where the control system is modelled with extended Petri nets and implemented as an asynchronous controller using circuit elements. The results provided in this paper on the digital hardware implementation of Petri nets may be view, as a better version of a previously introduced method [1], in terms of the implementation of transitions.Öğe Discrete event control system design using automation Petri nets and their ladder diagram implementation(SPRINGER LONDON LTD, 1998) Uzam, M; Jones, AHAs automated manufacturing systems become more complex, the need for an effective design tool to produce both high-level discrete event control systems (DECS) and low-level implementations becomes more important. Petri nets represent the most effective method for both the design and implementation of DECSs. In this paper automation Petri nets (APN) are introduced to provide a new method for the design and implementation of DECSs. The APN is particularly well suited to multiproduct systems and provides a more effective solution than Grafcet in this context. Since ordinary Petri nets do not deal with sensors and actuators of DECSs, the Petri net concepts are extended, by including actions and sensor readings as formal structures within the APN. Moreover enabling and inhibitor nrcs, which can enable or disable transitions through the use of leading-edge, falling-edge and level of markings, are also introduced. Tn this paper the methodology is explained by considering a fundamental APN structure. The conversion of APNs into the IEC1131-3 ladder diagrams (LD) for implementation on a PLC is also explained by using the token passing logic (TPL) concept Finally, an illustrative example of how APNs can be applied to a discrete manufacturing problem is described in detail.Öğe Iterative synthesis of Petri net based deadlock prevention policy for flexible manufacturing systems(IEEE, 2004) Uzam, M; Zhou, MCThis paper presents an iterative synthesis approach to Petri net based deadlock prevention policy for flexible manufacturing systems (FMS). Given the Petri net (PN) of an FMS prone to deadlock, the goal is to obtain a live controlled PN such that its use for control can lead to high utilization of system resources. In the proposed method, at each iteration, a first-met bad marking is singled out from the reachability graph of PN. The objective is to prevent this marking from being reached by a place invariant. To satisfy this place invariant, a well-established invariant-based control method is used to derive a control place with its related arcs and initial marking. This process is carried out until the PN becomes live. The method is easy to use, effective and straightforward It is generally applicable-yet its off-line computation is of exponential complexity. An example FMS is used to show the proposed method.Öğe Neurovision-based logic control of an experimental manufacturing plant using neural net le-net5 and automation Petri nets(SPRINGER, 2005) Karlik, B; Uzam, M; Cinsdikici, M; Jones, AHIn this paper, Petri nets and neural networks are used together in the development of an intelligent logic controller for an experimental manufacturing plant to provide the flexibility and intelligence required from this type of dynamic systems. In the experimental setup, among deformed and good parts to be processed, there are four different part types to be recognised and selected. To distinguish the correct part types, a convolutional neural net le-net5 based on-line image recognition system is established. Then, the necessary information to be used within the logic control system is produced by this on-line image recognition system. Using the information about the correct part types and Automation Petri nets, a logic control system is designed. To convert the resulting Automation Petri net model of the controller into the related ladder logic diagram (LLD), the token passing logic (TPL) method is used. Finally, the implementation of the control logic as an LDD for the real time control of the manufacturing system is accomplished by using a commercial programmable logic controller (PLC).Öğe Synthesis of feedback control elements for discrete event systems using Petri net models and theory of regions(SPRINGER LONDON LTD, 2004) Uzam, MThis paper describes a method for constructing a Petri-net-based controller for a discrete event system (DES) modelled by a Petri net. Assuming that an uncontrolled Petri net model of the DES and a set of forbidden state specifications are given, feedback control elements, i.e. a set of places and related transitions, with initial marking, are computed using the theory of regions, which is a formal synthesis technique for deriving Petri nets from automaton-based models. When feedback control elements are added to the uncontrolled Petri net model, the controlled (closed-loop) Petri net model of the system is obtained. The controlled Petri net model obtained is maximally permissive while guaranteeing that forbidden states do not occur. The proposed method is computationally efficient and does not suffer from the state explosion problem. Two examples are provided to show the applicability of the proposed method.Öğe The use of the Petri net reduction approach for an optimal deadlock prevention policy for flexible manufacturing systems(SPRINGER-VERLAG LONDON LTD, 2004) Uzam, MIn a flexible manufacturing system (FMS) with multiple products, deadlocks can arise due to limited shared resources, such as machines, robots, buffers, fixtures etc. The development of efficient deadlock prevention policies, which can optimise the use of system resources, while preventing deadlocks from occurring, has long been an important issue to be addressed. In [1], an optimal deadlock prevention policy was proposed, based on the use of reachability graph (RG) analysis of the Petri net model (PNM) of a given FMS and the synthesis of a set of new net elements, namely places with initial marking and related arcs, to be added to the PNM, using the theory of regions. The policy proposed in [1] is optimal in the sense that it allows the maximal use of resources in the system according to the production requirements. For very big PNMs, the reachability graph of the PNMs becomes very large and the necessary computations to obtain an optimal deadlock prevention policy become more difficult. In this paper, we propose the use of the Petri net reduction approach to simplify very big PNMs so as to make necessary calculations easily in order to obtain an optimal deadlock prevention policy for FMSs. An example is provided for illustration.Öğe Using a Petri-net-based approach for the real-time supervisory control of an experimental manufacturing system(SPRINGER LONDON LTD, 2000) Uzam, M; Jones, AH; Yucel, IA new Petri-net-based design technique, called the inhibitor arc method, for the synthesis of compiled supervisors for discrete event systems is used to solve a forbidden state problem in an experimental manufacturing system. The technique used offers the following advantages: 1. The closed-loop (i.e. controlled) behaviours of the systems are non-blocking and do not contradict the forbidden state specifications. 2. The closed-loop behaviours of the systems are maximally permissive within the specifications. The supervisors to be synthesised consist of a controlled automation Petri net (APN) model of the system. Automation Petri nets include the following extensions to the ordinary Petri-net framework: sensor readings as firing conditions at transitions and actions assigned to places. Ladder logic diagram (LLD) code is used to implement the supervisors on programmable logic controllers (PLC). It is important to note that the supervisors obtained are correct by construction, therefore there is no need for verification. This paper particularly shows the applicability of previous results [1] to low-level real-time control where the role of the supervisor is to arrange low-level interaction between the control devices, such as motors, actuators, etc. This is done by considering an experimental manufacturing system.
