Petri-net-based supervisory control of discrete event systems and their ladder logic diagram implementations

The last decade has witnessed rapid developments in computer technology, which inreturn, has found widespread applications in manufacturing systems, communicationnetworks, robots etc. Such systems are called Discrete Event Systems (DESs), in whichproperties such as non-determinism, conflict and parallelism are exhibited. As DESsbecome more complex, the need for an effective design tool and its implementationbecomes more important. Supervisory control theory, based on finite state machines(FSM) and formal languages, is a well established framework for the study of DESs. Insupervisory control, given a model of the system and the desired system behaviourspecifications, the objective is to find a supervisor (controller) such that the controlledbehaviour of the system does not contradict the specifications given and does notunnecessarily constrain the behaviour of the system. In general, the classes ofspecifications that have been considered within the supervisory control fall into twocategories: the forbidden state problem, in which the control specifications are expressedas forbidden conditions that must be avoided, and the desired string problem, in whichthe control specifications are expressed as sequence of activities that must be provided.In supervisory control, there are some problems when using FSMs as an underlyingmodelling tool. Firstly, the number of states grows exponentially as the system becomesbigger. Secondly, FMSs lack from graphical visivalisation. To overcome these problemsPetri nets have been considered as an alternative modelling tool for the analysis, designand implementation of such DESs, because of their easily understood graphicalrepresentation in addition to their well formed mathematical formalism.The thesis investigates the use of Petri nets in supervisory control. Both the forbiddenstate problem and the desired string problem are solved. In other words, this workpresents systematic approaches to the synthesis of Petri-nets-based supervisors(controllers) for both the forbidden state problem and the desired string problem andintroduces the details of supervisory design procedures. The supervisors obtained are the form of a net structure as oppose to supervisors given as a feedback fiinction. Thismeans that a controlled model of the system can be constructed and analysed using thetechniques regarding to Petri net models.In particular the thesis considers discrete manufacturing systems. The results obtainedcan be applied to high level control of manufacturing systems, where the role of thesupervisor is to coordinate the control of machines, robots, etc. and to low-level controlof manufacturing systems, where the role of the supervisor is to arrange low-levelinteractions between the control devices, such as motors, actuators, etc.An approach to the conversion from the supervisors to ladder logic diagrams (LLDs)for implementation on a programmable logic controller (PLC) is proposed. A discretemanufacturing system example is then considered. The aim of this is to illustrate theapplicability, strengths and drawbacks of the design techniques proposed

Synthesis of feedback control elements for discrete event systems using Petri net models and theory of regions

WOS: 000222862800007This 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

Ayrık olay sistemlerinde petri net temelli denetleyicilerin sentezi için yetkileyici ark metodu

Bu makalede, ayrık olay sistemlerindeki yasaklanmış durum problemini çözmek amacıyla Petri net temelli denetleyicilerin sentezi için yetkileyici ark metodu olarak adlandırılan yeni bir teknik önerilmiştir. Önerilen bu teknik şu avantajları sunmaktadır: i) sistemlerin kontrol edilmiş davranışı kısıtlamayıcıdır {nonblocking) ve yasaklanmış durum şartnameleri (forbidden state specifications) ile ters düşmez, ii) sistemlerin kontrol edilmiş davranışı şartnameler dahilinde maksimum müsaade edicidir (maximally permissive). Sentez edilecek olan denetleyiciler üç kısımdan oluşur: kontrol edilmemiş sistem davranışını temsil eden Otomasyon Petri Net (Automation Petri Net -APN) modeli, müsaade edilebilir maksimum sistem davranışını temsil eden APN model denetleyicisi ve bir kontrol planı (control policy). APN'ler alışılmış Petri net yapısına ek olarak şu ilaveleri içerirler: geçişlerde (transitions) ateşleme şartları olarak kullanılan sensör okumaları {sensor readings) ve ortamlara (places) atanmış olan faaliyetler {actions). Basamak mantık diyagramı (Ladder logic diagram - LLD) kodu, denetleyicileri programlanabilir mantık kontrolörlerinde (Programmable Logic Controllers - PLC) uygulamak için kullanılır. Önemle belirtilmelidir ki elde edilen deneteleyiciler yapı itibarı ile doğru olduğu için bir doğrulama işlemine ihtiyaç yoktur. Önerilen sentez tekniğinin uygulanabilirliği bir ayrık imalat sistemi incelenerek gösterilmiştir.In this paper, a new technique, called the enabling arc method, for the synthesis of Petri-net-based supervisors for Discrete Event Systems is proposed to solve the forbidden state problem. The technique proposed offers the following advantages: i) the controlled behaviour of the systems is nonblocking and does not contradict the forbidden state specifications, ii) the controlled behaviour of the systems is maximally permissive within the specifications. The supervisors to be synthesised consist of an Automation Petri Net (APN) model, which represents the uncontrolled behaviour of the system, an APN model supervisor, which represents the maximally permissible system behaviour, and a control policy. APNs 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. The applicability of the proposed synthesis technique is demonstrated by considering a discrete manufacturing system

The use of the Petri net reduction approach for an optimal deadlock prevention policy for flexible manufacturing systems

WOS: 000188765000008In 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

An optimal deadlock prevention policy for flexible manufacturing systems using Petri net models with resources and the theory of regions

WOS: 000174724700006In 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

Comments on "Feedback control logic for forbidden-state problems of marked graphs: Application to a real manufacturing system"

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A hybrid approach to supervisory control of discrete event systems coupling RW supervisors to Petri nets

WOS: 000236514400015In 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

An improved iterative synthesis method for liveness enforcing supervisors of flexible manufacturing systems

WOS: 000237065500007Our 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