Control Of Discrete Event Systems Research Articles

A survey on compositional algorithms for verification and synthesis in supervisory control

AbstractThis survey gives an overview of the current research on compositional algorithms for verification and synthesis of modular systems modelled as interacting finite-state machines. Compositional algorithms operate by repeatedly simplifying individual components of a large system, replacing them by smaller so-called abstractions, while preserving critical properties. In this way, the exponential growth of the state space can be limited, making it possible to analyse much bigger state spaces than possible by standard state space exploration. This paper gives an introduction to the principles underlying compositional methods, followed by a survey of algorithmic solutions from the recent literature that use compositional methods to analyse systems automatically. The focus is on applications in supervisory control of discrete event systems, particularly on methods that verify critical properties or synthesise controllable and nonblocking supervisors.

Language Recovery in Discrete-Event Systems against Sensor Deception Attacks

Cyber-physical systems are characterized by the intrinsic combination of software and physical components that usually include (wired and wireless) communication devices, sensors, actuators, and control processing units. Some wireless devices communicate over insecure channels, rendering cyber-physical systems at risk of malicious attacks that might lead to catastrophic damage. This paper touches upon the problem of sensor deception attacks in supervisory control of discrete-event systems, where an attacker can insert, delete, or replace sensor readings to mislead the supervisor and induce system damage. We model potential attacks using nondeterministic finite-state transducers and then introduce a new defence strategy that utilizes insertion functions. Insertion functions are a type of monitoring interface that alters the system’s behaviour by adding extra observable events. Finally, we construct a nondeterministic finite-state transducer called a supervisor filter that recovers the original language generated by the plant by handling the altered language. The insertion function and the supervisor filter cooperate to control the system and confuse the intruder without confusing the supervisor.

Detection of Actuator Enablement Attacks by Petri Nets in Supervisory Control Systems

The feedback control system with network-connected components is vulnerable to cyberattacks. We study a problem of attack detection in supervisory control of discrete-event systems. The scenario of a system subjected to actuator enablement attacks is considered in this article. We also consider that some unsafe places that should be protected from an attacker exist in the system, and some controllable events that are disabled by a supervisor might be re-enabled by an attacker. This article proposes a defense strategy to detect actuator enablement attacks and disable all controllable events after detecting an attack. We design algorithmic procedures to determine whether the system can be protected against damage caused by actuator enablement attacks, where the damage is predefined as a set of “unsafe” places. In this way, the system property is called “AE-safe controllability”. The safe controllability can be verified by using a basis diagnoser or a basis verifier. Finally, we explain the approach with a cargo system example.

ИНТЕЛЛЕКТУАЛЬНОЕ ПЛАНИРОВАНИЕ СТРАТЕГИЙ И УПРАВЛЕНИЕ ГРУППОЙ МОБИЛЬНЫХ РОБОТОВ В УСЛОВИЯХ НЕПОЛНОТЫ ИНФОРМАЦИИ

Different problems of strategy planning and control of a mobile robot group under complex dynamicconditions with incomplete information about the external environment are considered. Approachesto solving problems of effective work scheduling under conditions of inconstant active groupcomposition, searching for the source of a nonstationary concentration field, supervisory control ofdiscrete-event systems are presented. An original mathematical model formulated in terms of work-shiftscheduling problems and a problem-oriented modification of evolutionary algorithms with a specializedset of heuristics for its efficient solution are developed for the problem of scheduling top-level groupwork. Searching and monitoring the source of the nonstationary concentration field is carried out usinga decentralized multi-agent control strategy that combines elements of bionic and gradient approaches,as well as a method for generating artificial potential fields. The considered control strategy has lowcomputational complexity, high variability with respect to the types of fields surveyed, and is easily scalableto control any available number of mobile robots. The latter is of special importance, in particularwhen considering the problem of parallel and independent monitoring of multiple sources. It is proposedto use the means of logical inference, namely automatic theorem proving in the calculus of positivelyconstructed formulas, to solve various problems of the supervised control theory of discrete-eventsystems used at different levels of the robotic complex hierarchical control system. Features of the calculusallows solving complex problems of dynamic systems control, as well as processing and controllingevents based on environmental data in real time in the process of logical inference efficiently. Theapproach based on positively constructed formulas allows studying the properties of automata-baseddiscrete-event systems, as well as to synthesize and model finite automata for the construction and realizationof monolithic and modular supervisors. A general scheme combining the considered approachesfor controlling a group of mobile robots at different levels and time scales within a single hierarchicalcontrol system is proposed.

MDESops: An Open-Source Software Tool for Discrete Event Systems Modeled by Automata

The salient features of the new software tool MDESops are presented. MDESops is Python-based and open-source. Its focus is the analysis and control of discrete event systems (DES) modeled by finite-state automata. MDESops has core functions to manipulate deterministic and nondeterministic automata, including parallel composition and determinization. It has functions to analyze diagnosability and opacity properties. MDESops also includes functions that implement both standard and more recent algorithmic procedures from the theory of supervisory control of DES, including synthesis of supervisors for partially-observed systems, synthesis of attackers in systems with compromised sensors, and synthesis of supervisors resilient to deception attacks. It is the hope that MDESops can serve as a useful platform for algorithm development and prototyping in DES research.

Reducing the Learning Time of Reinforcement Learning for the Supervisory Control of Discrete Event Systems

Reducing the Learning Time of Reinforcement Learning for the Supervisory Control of Discrete Event Systems

UltraDES Project - A Multiplatform Discrete Event Systems Tool*

In this paper we present the UltraDES Project, composed of the UltraDES library, as well as a wrapper to Python and a Web Application. UltraDES is an open source project, available at https://github.com/lacsed/UltraDES. It has several functions and data structures for analysis and control of Discrete Event Systems and is under development since 2013. Currently, a new set of algorithms were implemented in the context of Supervisory Control Theory, but also some basic functions for Petri nets.

Max-Min-Plus-Scaling Systems in a Discrete-Event Framework with an Application in Urban Railway

In this paper we discuss modelling and control of discrete-event systems using max-min-plus-scaling systems. We analyse how the basic operations max, min, plus, and scaling occur in the modelling phase and we discuss some general forms for the system. Because of the different/deviating character of the signals in a discrete-event MMPS model, we will discuss concepts such as time-invariance and steady-state behavior. In the design of a model predictive controller for MMPS systems we have to revisit the cost functions in light of the discrete-event nature of the signals. We finalize this paper with the an intuitive case study on an urban railway line, performing both modelling and control.

Attack-Resilient Supervisory Control of Discrete-Event Systems: A Finite-State Transducer Approach

Attack-Resilient Supervisory Control of Discrete-Event Systems: A Finite-State Transducer Approach

Overview of networked supervisory control with imperfect communication channels

This paper presents an overview of the networked supervisory control frameworks for discrete event systems with imperfect communication networks, which are divided into the centralized setup, the decentralized setup and the distributed setup. The state-of-the-art works on networked supervisory control of discrete-event systems addressing the channel imperfections that are caused by channel delays, data losses or non-FIFO channels are discussed. By presenting the key concepts and main results of each representative work, we analyze the pros and cons of different approaches. Finally, we also provide a summary of the existing works, which roughly follow two different lines of thinking and result in two different verification or synthesis approaches. The first approach utilizes simple, non-networked plant models but relies on the development of sophisticated concepts of network controllability and observability to capture network imperfections, while the second approach embeds relatively complex yet verifiable channel models into the model of the networked plant and adopts the standard concepts of controllability and observability for the (verification and) synthesis of networked supervisors. Some future research topics are also presented.

Finite-Memory Supervisory Control of Discrete Event Systems for LTL[$\mathcal {F}$] Specifications

In this article, we study a supervisory control problem under a constraint on memory size as well as a constraint described by a class of quantitative linear temporal logic, which enables us to consider how well the specification is satisfied. Our control objective is to design a finite-memory supervisor such that the satisfaction value of the specification formula on the controlled system is larger than or equal to a given threshold. We adopt a Safraless synthesis methodology and reduce the problem to solving a safety game parameterized with the memory size of a supervisor. On the safety game, we compute a winning strategy by leveraging a ranking function. Our definition of the ranking function relies on the product automaton, which captures both the behavior of the controlled plant and that of an automaton transformed from the specification and the threshold.

On-the-fly informed search of non-blocking directed controllers

We study directed control of discrete event system expressed as the parallel composition of interacting automata. Solutions that first compose the automata and then compute a controller may result in an exponential blow up. We present a technique that builds the composition on-the-fly guided by a novel domain-independent heuristic, which attempts to discover relevant dependencies between the intervening components. We obtain safe and non-blocking directed controllers, or directors, exploring a reduced portion of the state space. We present the first experimental results on directed control comparing on-the-fly composition with informed search against the original monolithic approach to directed control.

A Discrete-Event System Approach for Modeling and Mitigating Power System Cascading Failures

A power system cascading failure can propagate through sequential tripping of components in the network. As a result, a complete or partial shutdown may occur. In this article, we develop a new systematic approach to identify and prevent cascading failures in power systems using supervisory control of discrete-event systems (DESs). We build the DES model for a power system in a modular fashion by first modeling its components as (small) automata and then combining these automata using parallel composition. To overcome state explosion, we use online lookahead control that can significantly reduce the number of states to be considered. Since some events such as line trips cannot be disabled but can be preempted by forcing some forcible events such as load shedding, we extend supervisory control of DES to include forcible events. This extension allows us to control power systems effectively. The proposed control is implemented in an implementation platform that we build in MATLAB. The platform uses MATPOWER to simulate a power system and then control it using the proposed DES controller. Simulation studies are carried out for IEEE 6-, 30-, and 118-bus systems. The results verify the effectiveness of the proposed approach.

Combining Online Diagnosis and Prognosis for Safe Controllability

In this article, we combine fault diagnosis and prognosis to generalize the notion of safe controllability of discrete-event systems. To do so, we reformulate the notions of safe diagnosability, prognosability, safe controllability by diagnosis, and safe controllability by prognosis in the context of strings. Moreover, we combine these notions to introduce the concept of safe controllability by diagnosis or prognosis, or simply DP-safe controllability. We show that a language can be DP-safe controllable even if it is not safe controllable either only by diagnosis or only by prognosis. Thus, the DP-safe controllability can be considered a generalization of the safe controllability concept found in the literature. If a DES is DP-safe controllable, to achieve fault tolerance using an active approach, reconfiguration actions could be forced based not only on online fault diagnosis, but also on online fault prognosis. Thus, our approach outperforms the previous ones, since it provides additional control options to keep the system away from forbidden zones and to switch from the nominal supervisor to a postfault-detection supervisor designed to achieve postfault performance objectives. Necessary and sufficient conditions for DP-safe controllability are presented and an example is used to illustrate the introduced concepts.

Using Subobservers to Synthesize Opacity-Enforcing Supervisors

In discrete-event system control, the worst-case time complexity for computing a system's observer is exponential in the number of that system's states. This results in practical difficulties since some problems require calculating multiple observers for a changing system, e.g., synthesizing an opacity-enforcing supervisor. Although calculating these observers in an iterative manner allows us to synthesize an opacity-enforcing supervisor and although methods have been proposed to reduce the computational demands, room exists for a practical and intuitive solution. Here we extend the subautomaton relationship to the notion of a subobserver and demonstrate its use in reducing the computations required for iterated observer calculations. We then demonstrate the subobserver relationship's power by simplifying state-of-the-art synthesis approaches for opacity-enforcing supervisors under realistic assumptions.

Educational Case Studies for Pilot Engineer 4.0 Programme: Monitoring and Control of Discrete-Event Systems Using OPC UA and Cloud Applications

The current trend in industry is the digitalisation of production processes using modern information and communication technologies, a trend that falls under the fourth industrial revolution, Industry 4.0. Applications that link the world of information technologies (IT) and operational technologies (OT) are in particular demand. On the basis of information from practice, it can be stated that there is a shortage of specialists in the labour market for the interconnection of PLCs with information and communication technologies (cloud, web, mobile applications, etc.) in Slovakia and neighbouring countries. However, this problem is beginning to affect other countries in Europe as well. The main objective of the work was to prepare case studies suitable for educational purposes, which would address the modelling and control of a virtual discrete-event system using a PLC program and its subsequent interfacing to a cloud application. Within the scope of the work, three case studies were prepared to demonstrate the control of discrete-event system using different programming systems and their communication with the developed cloud applications. These applications are to be used for data monitoring and emergency intervention of the discrete-event system. The characteristics of the prepared case studies, which combine operational and informational technologies, predestines them for use in the sphere of education of engineers for digitalisation of production processes. They can also be helpful in research on the creation of digital twins, which represent a type of symmetry between real and virtual systems.

Deterministic supervisory control with flexible upper-bounds on observation delay and control delay

This paper considers supervisory control of discrete event systems with observation delay and control delay. We first extend the formulation of networked supervisory control in the literature by allowing both the upper bound of observation delay and the upper bound of control delay to vary among different event strings, and accommodating dynamic observation in which the past influences whether or not an occurrence of an event is observable. We then combine observation delay and control delay and prove that satisfying both controllability and delay observability over the combined delay is necessary and sufficient for the existence of a supervisor by which the desired language can be obtained deterministically. After that, we reduce the type of problems concerning the networked supervisory control to the corresponding type of problems concerning traditional supervisory control without any delay on observation or control. As an immediate application, the verifier of observability can be used to test the corresponding delay observability. Finally, this reduction is used to leverage methods for minimizing sensor activations for supervisory control without control delay to minimize sensor activations with control delay.

Modular control of discrete-event systems using similarity

We investigate modular supervisory control of discrete-event systems composed of several groups of components, where each group consists of similar modules. Because of the similar structures of the modules, such systems can be represented as a set of (group) templates. Supervisory control can then be performed on these templates, resulting in a set of template supervisors. We propose a modular approach to construct the template supervisors based on the local computation of supremal symmetric sublanguages and on the concept of conditional decomposability. The supremal symmetric sublanguage of a decomposable language turns out to be decomposable, and can thus be computed locally. It is proven that the local supervisors of the components of a group are similar and can thus be obtained by a symmetry map from the template supervisor of the group.

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.

Reliable Decentralized Supervisory Control of Discrete Event Systems with Single-Level Inference

Reliable Decentralized Supervisory Control of Discrete Event Systems with Single-Level Inference