Networked Discrete Event Systems Research Articles

State-Based Opacity Verification of Networked Discrete Event Systems Using Labeled Petri Nets

State-Based Opacity Verification of Networked Discrete Event Systems Using Labeled Petri Nets

Information control in networked discrete event systems

How to control information exchange among different users is an important problem in networked systems with many users/agents. Generally speaking, there are several considerations in control of information exchange in a networked system, including (1) to ensure a friend user has sufficient information to perform its tasks, (2) to deprive an adversary user its information to perform its tasks, (3) to minimize information exchange among friend users so that the risk of information leaking is minimized, and (4) to maximize information broadcasted to all users to achieve maximum transparency. In this paper, we investigate the information control problems in the framework of discrete event systems. Based on the problem at hand, we divide users in a networked system into two or more groups. Users in the same group are consider as friends and users in a different group are consider as adversaries. Several information control problems are investigated and solved using a systematic and rigorous approach. Methods are developed to design controllers that send minimum information to its friends to help them to perform their tasks and broadcast maximum information without helping its adversaries.

Online State Estimation for Supervisor Synthesis in Discrete-Event Systems With Communication Delays and Losses

In the context of networked discrete-event systems (DESs), communication delays and losses exist between the plant and the supervisor for observation and between the supervisor and the actuator for control. In this paper, we first introduce a new framework for supervisory control of networked DESs. Under the introduced framework, we address the state estimation problem for supervisor synthesis of networked DESs with both communication delays and losses. The estimation algorithm considers the effect of the controls imposed on the system. Additionally, the estimation algorithm is based on the control decisions available up to the moment, and all the future control decisions are assumed to be unknowable. Two notions, called “observation channel configuration” for tracking observation delays and losses and “control channel configuration” for tracking control delays and losses, are defined. Then, we introduce an online approach for state estimation of the controlled system. Compared with the existing approach, the proposed approach under the introduced framework can estimate the states of the controlled system more accurately. As an application of the proposed approach, we finally show that the existing methods can be applied to synthesize maximally permissible and safe networked supervisors.

Minimization of sensor activation in discrete-event systems with control delays and observation delays

In discrete-event systems, to save sensor resources, the agent continuously adjusts sensor activation decisions according to a sensor activation policy based on the changing observations. However, new challenges arise for sensor activation in networked discrete-event systems, where observation delays and control delays exist between the sensor systems and the agent. In this paper, a new framework for activating sensors in networked discrete-event systems is established. In this framework, we construct a communication automaton that explicitly expresses the interaction process between the agent and the sensor systems over the observation channel and the control channel. Based on the communication automaton, we can define dynamic observations of a communicated string. To guarantee that a sensor activation policy is physically implementable and insensitive to non-deterministic control delays and observation delays, we further introduce the definition of delay feasibility. We show that a delay feasible sensor activation policy can be used to dynamically activate sensors even if control delays and observation delays exist. A set of algorithms are developed to minimise sensor activation in a transition-based domain while ensuring a given specification condition is satisfied. A practical example is also provided to show the application of the proposed framework. Finally, we briefly discuss how to extend the proposed framework to a decentralised observation setting.

State Estimation and Detectability of Networked Discrete Event Systems With Multi-Channel Communication Networks

In this paper, we extend existing results on detectability of Discrete Event Systems (DES) to Networked Discrete Event Systems (NDES), a class of DES whose communication between the plant and the agent (supervisor, diagnoser, prognoser, etc) is carried out through a network that can have several channels. We assume that each channel is subject to different communication delays, which may cause changes in the order of the event observations with respect to their actual occurrences in the system automaton, and also that event transmission may be lost in the communication channels causing loss of observations. We follow a previously presented approach to construct an equivalent nondeterministic untimed automaton, and, based on this model, we present a method for the state estimation of NDES. We extend existing D-detectability definitions of DES to NDES subject to delays and loss of observation, referred here to as strong networked D-detectability, weak networked D-detectability, strong periodic networked D-detectability, weak periodic networked D-detectability. Finally, necessary and sufficient conditions for the networked D-detectability notions proposed here are proposed and algorithms for their verification are developed. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Note to Practitioners</i> —State estimation is an important problem that appear in several practical applications, e.g. industrial systems, aircraft control and business management. Of current interest is the problem of distinguishing in real time if the system is not in pairs of states, the so-called D-detectability. In practical terms, it is not necessary to actually know the current state of the system but if the system may not be in two or more conflicting states. With the use of communication networks, additional features such as communication delay must also be taken into account in the process of performing D-detectability. In this paper we consider discrete event systems, a class of systems used to model a wide class of practical engineering problems whose dynamic evolution is driven by event occurrences, that are embedded in a communication network; we will refer to these systems as networked discrete event systems. In this regard, we generalize existing notions of D-detectability, and we call them networked D-detectability. Algorithms for networked D-detectability verification and a method to perform online state estimation are given in the paper.

Supervisory Control of Networked Discrete Event Systems to Achieve a Required Language

Supervisory Control of Networked Discrete Event Systems to Achieve a Required Language

Minimal Sensor Activation for Detectable Networked Discrete Event Systems

Minimal Sensor Activation for Detectable Networked Discrete Event Systems

Covert Attack Synthesis for Networked Discrete-Event Systems*

The problem of covert sensor-actuator attack synthesis in the context of network discrete-event system (DES) is studied, in which the observation channel and control channel have bounded delays. We investigate a kind of attack that can 1) keep hidden, 2) implement sensor insertion, deletion and replacement attacks, and actuator enablement and disablement attacks. We propose a transformation-based method to convert the covert attack synthesis problem of network DES into the standard Ramadge-Wonham supervisory control problem. Our method is applicable to the damage-reachable goal and damage-nonblocking goal.

K-corruption Intermittent Attacks for Violating the Diagnosability

This work addresses the problem of diagnosability analysis of a networked discrete event system under external malicious attacks, in which the considered system is modeled by labeled Petri net, and is attacked by a given replacement structure (i.e., the original transition label is replaced by a different label or empty string). In addition, we assume that the attack is stealthy and intermittent, which requires that the corrupted observations should belong to the normal behavior of the system, and the replaced transition labels must be recovered within a bounded of consecutive corrupted observations (called as K-corruption intermittent attack), respectively. The aim of this work is, from an attacker viewpoint, to design a K-corruption intermittent attacks for violating the diagnosability of systems. To this end, an attack automaton and complete unfolded verifier, i.e., a special automaton, are established to analyze all the potential attacked paths. Then an algorithm is proposed to obtain the K-corruption intermittent attacks, and examples are used to illustrate the proposed attack method.

Modeling and Optimal Supervisory Control of Networked Discrete-Event Systems and Their Application in Traffic Management

In this paper, we investigate the modeling and control of networked discrete-event systems (DESs), where a supervisor is connected to the plant via an observation channel and the control commands issued by the supervisor are delivered to the actuator of the plant via a control channel. Communication delays exist in both the observation channel and the control channel. First, a novel modeling framework for the supervisory control of DESs subject to observation delays and control delays is presented. The framework explicitly models the interaction process between the plant and the supervisor over the communication channels. Compared with the previous work, a more accurate “dynamics” of the closed-loop system is specified. Under this framework, we further discuss how to estimate the states of the closed-loop system in the presence of observation delays and control delays. Based on the state estimation, we synthesize an optimal supervisor on the fly to maximize the controlled behaviors while preventing the system from leaving the desired behaviors under communication delays. We compare the proposed supervisor with the supervisor proposed in the literature and show that the proposed supervisor is more permissive. As an application, we show how the proposed approach can be applied to manage vehicles in a signal intersection. Finally, we show how to extend the proposed framework to model a system whose actuators and sensors are distributed at different sites.

Cooperative control of networked discrete‐event systems: Application to the handling system HANS

A method for the cooperative control of networked discrete-event systems is evaluated experimentally on a handling system. Networked discrete-event systems consist of controlled subsystems, which have to solve local and cooperative tasks. Local tasks are specified by target states of each individual subsystem, whereas cooperative tasks are modelled by synchronous state transitions for neighbouring subsystems. Network units connect the controlled subsystems via a digital network and apply situation dependent communication, if a cooperation is necessary. The experiments show that the proposed method is suitable to provide a flexibly structured handling system and that it is feasible with regard to the computational complexity.

Codiagnosability of Networked Discrete Event Systems With Timing Structure

We address, in this article, the problem of codiagnosability of networked discrete event systems with timing structure (NDESWTS) subject to delays and loss of observations of events between the measurement sites (MS) and local diagnosers (LD). To this end, we first introduce a new timed model that represents the dynamic behavior of the plant based on the, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a priori</i> , knowledge of the minimal activation time for each transition of the plant and on the maximal delays in the communication channels that connect MS and LD. We then convert this timed model into an equivalent untimed one, and add possible intermittent packet loss in the communication network. Based on this untimed model, we present necessary and sufficient conditions for NDESWTS codiagnosability and propose two tests for its verification: one that deploys diagnosers and another one that uses verifiers.

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.

Event-Based Fault Diagnosis of Networked Discrete Event Systems

The problem of fault diagnosis has become an active research topic in Discrete Event Systems, which can be applied into many different areas, such as communication networks, process control, flexible manufacturing systems, software engineering, etc. This brief explores the effect of communication delays on the event-based fault diagnosis in networked discrete event systems (NDESs), which can be modeled by finite state automata. Firstly, the framework of NDESs is introduced, and a novel concept of networked diagnosability ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Net$ </tex-math></inline-formula> -diagnosability) is formally defined. Subsequently, the necessary and sufficient conditions for the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Net$ </tex-math></inline-formula> -diagnosability in the partially observable outputs are provided, and the corresponding <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Net$ </tex-math></inline-formula> -diagnosability can be verified by an effective diagnoser, which can be obtained by a constructed algorithm. Finally, an illustrative example is given to demonstrate the proposed method. The current results about networked diagnosability will be conducive to maintain the information security in the actual cyber-physical systems.

I-Detectability of Networked Discrete Event Systems by Matrix Approach

I-Detectability of Networked Discrete Event Systems by Matrix Approach

Weak Diagnosability of Discrete-Event Systems

Abstract In this paper, we propose a weaker property for failure diagnosis called weak diagnosability. Weak diagnosability says that, after the occurrence of a fault event, for any continuation, there always exists at least one trajectory of sufficient length along which we can diagnose the occurrence of the fault event. We derive a necessary and sufficient condition for weak diagnosability and develop an algorithm to check the condition. We then extend weak diagnosability to networked discrete event systems which have packet losses and communication delays and call the extension weak network diagnosability. We adopt the transformed automaton technique to deal with packet losses and develop a method for checking weak network diagnosability with respect to packet losses. For communication delays, our results show that they do not change weak network diagnosability when delays are first-in-first-out, but may add time to diagnose the occurrences of fault events.

Robust decentralized diagnosability of networked discrete event systems against DoS and deception attacks

Denial-of-Service (DoS) are attacks conducted by malicious agents that consists in disrupting, temporally or indefinitely, the services provided by a communication network. When a malicious agent gets access to some network node, it may also perform deception attacks by inserting valid packets with fake information into vulnerable channels. We address, in this paper, DoS and deception attacks (DoS-D attack) that flood some communication channels with fake packets causing delays, loss of observations and insertion of fake observations, and their implications in decentralized fault diagnosability of networked discrete event systems (NDES). To this end, we propose an automaton model for NDES subject to DoS-D attacks that represents the adverse effects of DoS-D attacks on the observations of local diagnosers. We introduce a new codiagnosability definition called DoS-D-robust codiagnosability, and present a necessary and sufficient condition for a language to be DoS-D-robustly codiagnosable. We also propose a verification algorithm for regular languages to check DoS-D-robust codiagnosability.

A new modeling framework for networked discrete-event systems

This paper proposes a new framework for modeling networked discrete-event systems (DES) with channel delays and losses. We construct a more elaborated model for the plant G and supervisor S by endowing them with an embedded module to preprocess the received messages and defining state transition mapping and output mapping to model the dynamics of G and S, which could provide users with flexibility to realize different mechanisms, and thus could potentially generate more permissive controllers. Two sets of new formulations are established to capture the temporal relationships between channel inputs and outputs without assuming the first-in-first-out (FIFO) property, which naturally contains the model without delays and losses in the Ramadge–Wonham framework. Based on the defined mappings of each component, the closed-loop behavior is defined to describe the concurrent system dynamics. We hope the new framework could serve as an analysis tool, and possibly the building block for a synthesis tool, for future theoretical explorations and practical implementations of networked DES control.

Design of privacy-preserving control strategy for data opacity of industrial internet

<p indent="0mm">With the continuous development of information and communication technologies (ICT) as well as their applications in the industrial environment, industrial internet comes into being. In comparison to the relatively closed and secure production environment of traditional industries, the industrial internet faces significant security challenges in terms of data privacy. This paper investigates the privacy-preserving control problem for data opacity of the industrial internet to provide a resilient defending strategy against cyber threats, where the system to be controlled is modeled as a networked discrete-event system (DES) and malicious eavesdroppers exist to infer the system secret based on its observations. We here aim at designing a networked supervisor to achieve the following two objectives: (1) eavesdroppers cannot infer the system secret based on its observations, which is formulated as the requirement of opacity enforcement, (2) under the control of the designed networked supervisor, the system could still achieve the predetermined control requirement. To solve the previously mentioned privacy-preserving supervisory control problem, this paper first proposes a privacy-preserving control architecture for networked DES, in which each component is modeled as a finite-state automaton to simulate its dynamics. We design an algorithm based on the constructed models to synthesize the networked supervisor by transforming the original privacy-preserving control problem into a classical Ramadge-Wonham supervisory control problem. Furthermore, this paper proves (1) the designed synthesis algorithm is sound and complete, and (2) the existence of the supremal networked supervisor for this privacy-preserving supervisory control problem.

Online Supervisory Control of Networked Discrete Event Systems With Control Delays

We investigate state estimation and safe controller synthesis for networked discrete-event systems (DESs), where supervisors send control decisions to plants via communication channels subject to communication delays. Previous works on state estimation of networked DES are based on the open-loop system without utilizing the knowledge of the control policy. In this article, we propose a new approach for online estimation and control of networked DES with control delays. We first propose a new state estimation algorithm for the closed-loop system utilizing the information of control decision history. The proposed state estimation algorithm can be implemented recursively upon the occurrence of each new observable event. Then we investigate how to predict the effect of control delays in order to calculate a control decision online at each instant. We show that the proposed online supervisor can be updated effectively and the resulting closed-loop behavior is safe. Furthermore, we compare the proposed online supervisor with the predictive supervisor proposed in the literature and show that our proposed online supervisor is more permissive than predictive supervisor in the sense of language inclusion.