Nonconflict check by using sequential automaton abstractions

In Ramadge-Wonham supervisory control theory we often need to check nonconflict of plants and corresponding synthesized supervisors. For a large system such a check imposes a great computational challenge because of the complexity incurred by composition of plants and supervisors. In this paper we present a novel procedure based on automaton abstractions, which removes internal transitions of relevant automata at each step, allowing the nonconflict check to be performed over relatively small automata, even though the original system can be fairly large

Compositional nonblocking verification with always enabled events and selfloop-only events

This paper proposes to improve compositional nonblocking verification through the use of always enabled and selfloop-only events. Compositional verification involves abstraction to simplify parts of a system during verification. Normally, this abstraction is based on the set of events not used in the remainder of the system, i.e., in the part of the system not being simplified. Here, it is proposed to exploit more knowledge about the system and abstract events even though they are used in the remainder of the system. Abstraction rules from previous work are generalised, and experimental results demonstrate the applicability of the resulting algorithm to verify several industrial-scale discrete event system models, while achieving better state-space reduction than before

Generalised verification of the observer property in discrete event systems

The observer property is an important condition to be satisfied by abstractions of Discrete Event Systems (DES) models. This paper presents a generalised version of a previous algorithm which tests if an abstraction of a DES obtained through natural projection has the observer property. The procedure called OP-verifier II overcomes the limitations of the previously proposed verifier while keeping its computational complexity. Results are illustrated by a case study of a transfer line system

Generalised verification of the observer property in discrete event systems

The observer property is an important condition to be satisfied by abstractions of Discrete Event Systems (DES) models. This paper presents a generalised version of a previous algorithm which tests if an abstraction of a DES obtained through natural projection has the observer property. The procedure called OP-verifier II overcomes the limitations of the previously proposed verifier while keeping its computational complexity. Results are illustrated by a case study of a transfer line system

On Conditional Decomposability

The requirement of a language to be conditionally decomposable is imposed on a specification language in the coordination supervisory control framework of discrete-event systems. In this paper, we present a polynomial-time algorithm for the verification whether a language is conditionally decomposable with respect to given alphabets. Moreover, we also present a polynomial-time algorithm to extend the common alphabet so that the language becomes conditionally decomposable. A relationship of conditional decomposability to nonblockingness of modular discrete-event systems is also discussed in this paper in the general settings. It is shown that conditional decomposability is a weaker condition than nonblockingness.Comment: A few minor correction

Modular Verification and Supervisory Controller Design for Discrete-Event Systems Using Abstraction and Incremental Construction.

The subject of this dissertation is modular approaches to the verification and control of discrete-event systems (DES). DES are dynamic systems characterized by discrete states and event-driven evolution. In recent years, a substantial body of work has been built up to provide a theory and framework for the control and verification of DES. Despite all the advancements that have been made in this area, application to real-life systems has been somewhat slow. A significant hurdle to the adoption of these methods is the state-space explosion that occurs in modeling systems of the size most commonly found in industry. A common approach that has been applied to address this complexity problem is to construct a series of smaller modular supervisors, rather than a single monolithic supervisor. The problem with this approach is that the modular supervisors can often conflict with one another. This dissertation develops three new approaches to the supervisory control of DES that adopt a modular aspect to their control, while addressing the potential problem of conflict. The first approach addresses the problem of state-space explosion by offering a procedure for incrementally building modular supervisors that are guaranteed to not conflict with one another by construction. An observer type abstraction is employed to make the procedure more computationally feasible. The second approach of this dissertation constructs traditional modular supervisors, then adds another level of coordinating control to resolve conflict between the supervisors. This work employs a conflict-equivalence preserving abstraction to detect and resolve the conflict. The final approach of this dissertation employs interfaces between different components of the global system. The additional structure of these interfaces allows global properties to be verified through the achievement of local properties. Additionally, these interfaces allow for modular supervisors to be synthesized locally such that the necessary requirements are met by construction. In this work, the correctness of the three approaches is proven. Additionally, application to some manufacturing based examples are employed to illustrate the potential strengths and weaknesses of each of the approaches.Ph.D.Mechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/60669/1/rchill_1.pd

An algorithm for compositional nonblocking verification using special events

This paper proposes to improve compositional nonblocking verification of discrete event systems through the use of special events. Compositional verification involves abstraction to simplify parts of a system during verification. Normally, this abstraction is based on the set of events not used in the remainder of the system, i.e., in the part of the system not being simplified. Here, it is proposed to exploit more knowledge about the remainder of the system and check how events are being used. Always enabled events, selfloop-only events, failing events, and blocked events are easy to detect and often help with simplification even though they are used in the remainder of the system. Abstraction rules from previous work are generalised, and experimental results demonstrate the applicability of the resulting algorithm to verify several industrial-scale discrete event system models, while achieving better state-space reduction than before

Compositional nonblocking verification with always enabled and selfloop-only events

This report proposes to improve compositional nonblocking verification through the use of two special event types: always enabled and selfloop-only events. Compositional verification involves abstraction to simplify parts of a system during verification. Normally, this abstraction is based on the set of events not used in the remainder of the system. Here, it is proposed to exploit more knowledge about the system and abstract events even though they are used in the remainder of the system. This can lead to more simplification than was previously possible. Abstraction rules from previous work are extended to respect the new special events and proofs show these rules still preserve nonblocking. The rules have been implemented in Waters and experimental results demonstrate that these extended simplification rules help verify several industrial-scale discrete event system models while achieving better state-space reduction than before

On Conflicts in Concurrent Systems

This dissertation studies conflicts. A conflict is a bug in concurrent systems where one or more components of the system may potentially be blocked from completing their task. This dissertation investigates how nonconflicting completions may be used to characterise the situations in which individual components of a system may be in conflict with other components. The first major contributions of this dissertation are new methods of abstracting systems with respect to conflicts, and showing how these methods may be used to check whether a large system is conflict-free. The second contribution is a method of comparing whether one system is less susceptible to conflict than another. The last major contribution is a method of expressing all conflicts in a system in a finite and canonical way. The methods developed have applications for model checking, refinement, and the development of contracts for concurrent systems