Distributed Supervisory Control of Discrete-Event Systems with Communication Delay

This paper identifies a property of delay-robustness in distributed supervisory control of discrete-event systems (DES) with communication delays. In previous work a distributed supervisory control problem has been investigated on the assumption that inter-agent communications take place with negligible delay. From an applications viewpoint it is desirable to relax this constraint and identify communicating distributed controllers which are delay-robust, namely logically equivalent to their delay-free counterparts. For this we introduce inter-agent channels modeled as 2-state automata, compute the overall system behavior, and present an effective computational test for delay-robustness. From the test it typically results that the given delay-free distributed control is delay-robust with respect to certain communicated events, but not for all, thus distinguishing events which are not delay-critical from those that are. The approach is illustrated by a workcell model with three communicating agents

Robust decentralized supervisory control of discrete-event systems

In this thesis we study robust supervisory control of discrete event systems in two different settings. First, we consider the problem of synthesizing a set of decentralized supervisors when the precise model of the plant is not known, but it is known that it is among a finite set of plant models. To tackle this problem, we form the union of all possible behaviors and construct an appropriate specification, from the given set of specifications, and solve the conventional decentralized supervisory control associated with it. We also prove that the given robust problem has a solution if and only if this conventional decentralized supervisory control problem has a solution. In another setting, we investigate the problem of synthesizing a set of communicating supervisors in the presence of delay in communication channels, and call it Unbounded Communication Delay Robust Supervisory Control problem (UCDR-SC problem). In this problem, We assume that delay is unbounded but it is finite, meaning that any message sent from a local supervisor will be received by any other local supervisors after a finite but unknown delay. To solve this problem, we redefine the supervisory decision making rules, introduce a new language property called unbounded-communication-delay-robust (UCDR), and present a set of conditions on the specification of the problem. We also show that the new class of languages that is the solution to this problem has some interesting relations with other observational languages

Controller Synthesis for Bisimulation Equivalence

Ph.DDOCTOR OF PHILOSOPH

Networked Supervisor Synthesis Against Lossy Channels with Bounded Network Delays as Non-Networked Synthesis

In this work, we study the problem of supervisory control of networked discrete event systems. We consider lossy communication channels with bounded network delays, for both the control channel and the observation channel. By a model transformation, we transform the networked supervisor synthesis problem into the classical (non-networked) supervisor synthesis problem (for non-deterministic plants), such that the existing supervisor synthesis tools can be used for synthesizing networked supervisors. In particular, we can use the (state-based) normality property for the synthesis of the supremal networked supervisors, whose existence is guaranteed by construction due to our consideration of command non-deterministic supervisors. The effectiveness of our approach is illustrated on a mini-guideway example that is adapted from the literature, for which the supremal networked supervisor has been synthesized in the synthesis tools SuSyNA and TCT.Comment: This paper is under review for Automatic

Control and diagnosis of real-time systems under finite-precision measurement of time

A discrete event system (DES) is an event-driven system that evolves according to abrupt occurrences of discrete changes (events). The domain of such systems encompasses aspects of many man-made systems such as manufacturing systems, telephone networks, communication protocols, traffic systems, embedded software, asynchronous hardware, robotics, etc. Supervisory control theory for DESs studies the existence and synthesis of the supervisory controllers, namely, supervisors that restrict the system behaviors by dynamically disabling certain controllable events so that the controlled close-loop system could behave as desired. Extensive work on supervisory control of untimed DESs exists and the extension to the timed setting has been reported in the literature. In this dissertation, we study the supervisory control of dense-time DESs in which the digital-clocks of finite-precision are employed to observe the event occurrence times, thereby relaxing the assumption of the prior works that time can be measured precisely. In our setting, the passing of time is measured using the number of ticks generated by a digital-clock and we allow the plant events and digital-clock ticks to occur concurrently. We formalize the notion of a control policy that issues the control actions based on the observations of events and their occurrence times as measured using a digital-clock, and show that such a control policy can be equivalently represented as a digitalized -automaton, namely, an untimed-automaton that evolves over the events (of the plant) and ticks (of the digital-clock). We introduce the notion of observability with respect to the partial observations of time resulting from the use of a digital-clock, and show that this property together with controllability serves as a necessary and sufficient condition for the existence of a supervisor to enforce a real-time specification on a dense-time discrete event plant. The observability condition presented in the dissertation is very different from the one arising due to a partial observation of events since a partial observation of time is in general nondeterministic (the number of ticks generated in any time interval can vary from execution to execution of a digital-clock). We also present a method to verify the proposed observability and controllability conditions, and an algorithm to compute a supervisor when such conditions are satisfied. Furthermore we examine the lattice structure of a class of timing-mask observable languages, and show that the proposed observability is not preserved under intersection but preserved under union. Fault diagnosis for DESs is to detect the occurrence of a fault so as to enable any corrective actions. It is crucial in automatic control of large complex man-made systems and has attracted considerable attention in the literature of reliability engineering, control and computer science. For the event-driven systems with timing-requirements such as manufacturing systems, communication networks, real-time scheduling and traffic systems, fault diagnosis involves detecting the timing-faults, besides the sequence-faults. This requires monitoring timing and sequence of events, both of which may only be partially observed in practice. In this dissertation, we extend the prior works on fault diagnosis of timed DESs by allowing time to be partially observed using a digital-clock which measures the advancement of time with finite precision by the number of ticks. For the diagnosis purposes, the set of nonfaulty timed-traces is specified as another timed-automaton that is deterministic. We show that the set of timed-traces observed using a digital-clock with finite precision is regular, i.e., can be represented using a finite (untimed) automaton. We also show that the verification of diagnosability (the ability to detect the execution of a faulty timed-trace within a bounded time delay) as well as the off-line synthesis of a diagnoser are decidable by reducing these problems to the untimed setting. The reduction to the untimed setting also suggests an effective method for the off-line computation of a diagnoser as well as its on-line implementation for diagnosis. The aforementioned results are further extended to the nondeterministic setting, i.e., diagnosis of dense-time DESs using digital-clocks under nondeterministic event observation mask. We introduce the notion of lifting (associating each event with each of its nondeterministic observations), and show that diagnosis of dense-time DESs employing digital-clocks to observe event occurrence times under nondeterministic event observation mask can be reduced to that of the deterministic setting, i.e., diagnosis of the lifted dense-time DESs under the deterministic lifted event observation mask, and hence can be further reduced to diagnosis of the untimed setting

Synthesis of communicating decentralized supervisors for discrete-event systems with application to communication protocol synthesis

A Discrete-Event Systems (DES) may be viewed as a dynamic system with a discrete state space and a discrete state-transition structure with an event-driven nature, which makes it different from the systems described by differential or difference equations. Given the desired behavior of a DES as a specification, decentralized supervisory control theory seeks to design for a (distributed) DES, consisting of a number of (geographically distant) sites, a set of supervisors, one for each site, such that the behavior of the DES always remains within the specification. If the specification is not coobservable, these supervisors need to communicate amongst each other. This thesis proposes a mathematical framework to formally model and synthesize such communicating decentralized supervisors. The framework provides a decentralized representation of the DES's centralized supervisor and captures its observational and control-related information as mappings, which are called updating and guard functions, respectively. This leads to a polynomial dynamical system, which serves to model the required communication and synthesize its rules. The systematic synthesis, obtained through this approach, characterizes the class of distributed control problems which are solvable only with communication, comes up with a finer partition of it, and addresses practical issues. The thesis ends with the application of the theoretical results to the modeling and synthesis of a communication protoco

Water in the Arab World

This volume is intended to serve as a water handbook. It represents the collective knowledge about water resources management acquired over recent years, both within the World Bank water team and with counterparts working in the Arab countries of North Africa and the Middle East (MNA). The chapters offer a cornucopia of ideas and themes. Some chapters are based on background papers prepared for the 2007 "MNA Development Report on Water." Others draw on sector work prepared at the request of client countries. Yet others summarize observations based on study tours or other learning events sponsored by the World Bank. Upon reviewing this lodestone of embedded knowledge, we realized that bringing together our observations and analyses could serve a useful purpose for public officials, other practitioners, academics, and students who are interested in learning more about the complexities of managing water resources management in one of the driest parts of the world

Computer Aided Verification

This open access two-volume set LNCS 11561 and 11562 constitutes the refereed proceedings of the 31st International Conference on Computer Aided Verification, CAV 2019, held in New York City, USA, in July 2019. The 52 full papers presented together with 13 tool papers and 2 case studies, were carefully reviewed and selected from 258 submissions. The papers were organized in the following topical sections: Part I: automata and timed systems; security and hyperproperties; synthesis; model checking; cyber-physical systems and machine learning; probabilistic systems, runtime techniques; dynamical, hybrid, and reactive systems; Part II: logics, decision procedures; and solvers; numerical programs; verification; distributed systems and networks; verification and invariants; and concurrency