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

Strict Minimal Siphon-Based Colored Petri Net Supervisor Synthesis for Automated Manufacturing Systems With Unreliable Resources

Various deadlock control policies for automated manufacturing systems with reliable and shared resources have been developed, based on Petri nets. In practical applications, a resource may be unreliable. Thus, the deadlock control policies proposed in previous studies are not applicable to such applications. This paper proposes a two-step robust deadlock control strategy for systems with unreliable and shared resources. In the first step, a live (deadlock-free) controlled system that does not consider the failure of resources is derived by using strict minimal siphon control. The second step deals with deadlock control issues caused by the failures of the resources. Considering all resource failures, a common recovery subnet based on colored Petri nets is proposed for all resource failures in the Petri net model. The recovery subnet is added to the derived system at the first step to make the system reliable. The proposed method has been tested using an automated manufacturing system deployed at King Saud University.publishedVersio

The planning coordinator: A design architecture for autonomous error recovery and on-line planning of intelligent tasks

Developing a robust, task level, error recovery and on-line planning architecture is an open research area. There is previously published work on both error recovery and on-line planning; however, none incorporates error recovery and on-line planning into one integrated platform. The integration of these two functionalities requires an architecture that possesses the following characteristics. The architecture must provide for the inclusion of new information without the destruction of existing information. The architecture must provide for the relating of pieces of information, old and new, to one another in a non-trivial rather than trivial manner (e.g., object one is related to object two under the following constraints, versus, yes, they are related; no, they are not related). Finally, the architecture must be not only a stand alone architecture, but also one that can be easily integrated as a supplement to some existing architecture. This thesis proposal addresses architectural development. Its intent is to integrate error recovery and on-line planning onto a single, integrated, multi-processor platform. This intelligent x-autonomous platform, called the Planning Coordinator, will be used initially to supplement existing x-autonomous systems and eventually replace them

Modeling and analysis of semiconductor manufacturing processes using petri nets

This thesis addresses the issues in modeling and analysis of multichip module (MCM) manufacturing processes using Petri nets. Building such graphical and mathematical models is a crucial step to understand MCM technologies and to enhance their application scope. In this thesis, the application of Petri nets is presented with top-down and bottom-up approaches. The theory of Petri nets is summarized with its basic notations and properties at first. After that, the capability of calculating and analyzing Petri nets with deterministic timing information is extended to meet the requirements of the MCM models. Then, using top-down refining and system decomposition, MCM models are built from an abstract point to concrete systems with timing information. In this process, reduction theory based on a multiple-input-single-output modules for deterministic Petri nets is applied to analyze the cycle time of Petri net models. Besides, this thesis is of significance in its use of the reduction theory which is derived for timed marked graphs - an important class of Petri nets

On the Enforcement of a Class of Nonlinear Constraints on Petri Nets

International audienceThis paper focuses on the enforcement of nonlinear constraints in Petri nets. First, a supervisory structure is proposed for a nonlinear constraint. The proposed structure consists of added places and transitions. It controls the transitions in the net to be controlled only but does not change its states since there is no arc between the added transitions and the places in the original net. Second, an integer linear programming model is proposed to transform a nonlinear constraint to a minimal number of conjunc-tive linear constraints that have the same control performance as the nonlinear one. By using a place invariant based method, the obtained linear constraints can be easily enforced by a set of control places. The control places consist to a supervisor that can enforce the given nonlinear constraint. On condition that the admissible markings space of a nonlinear constraint is non-convex, another integer linear programming model is developed to obtain a minimal number of constraints whose disjunctions are equivalent to the nonlinear constraint. Finally, a number of examples are provided to demonstrate the proposed approach

Petri net approaches for modeling, controlling, and validating flexible manufacturing systems

In this dissertation, we introduce the fundamental ideas and constructs of Petri net models such as ordinary, timed, colored, stochastic, control, and neural, and present some studies that emphasize Petri nets theories and applications as extended research fields that provide suitable platforms in modeling, controlling, validating, and evaluating concurrent systems, information systems, and a versatile dynamic system and manufacturing systems;We then suggest some of extensions that help make Petri nets useful for modeling and analyzing discrete event systems and manufacturing systems models based on the context of a versatile manufacturing system, and applies extended Petri nets models to several manufacturing systems such as an assembly cell, an Automated Palletized Conveyor System, and a tooling machine to show increased modeling power and efficient analysis methods;Finally, Validation methods are presented for these models and results of a performance analysis from a deterministic and stochastic model are used to reorganize and re-evaluate a manufacturing system in order to increase its flexibility