Networked Discrete Event Systems Research Articles

Supervisory Control of Networked Discrete Event Systems With Timing Structure

In this article, we study the supervisory control problem of networked discrete event systems with timing structure assuming bounded communication delays and intermittent loss of observations. The communication between the plant and the supervisor is through a network that can have several channels, and so communication delays may change the order of event observations by the supervisor. We assume a priori knowledge of the minimal activation time of the plant transitions and the maximum communication delays, and propose an equivalent untimed model that takes into account all possible observation delays and also loss of observations. Based on this model, we formulate a networked supervisory control problem and show that it can be converted into an equivalent range control problem. We also address implementation, and, to this end, we present a representation for networked supervisors that is able to deal with possible loss of synchronization between event occurrence and its observation.

Opacity of Networked Supervisory Control Systems Over Insecure Communication Channels

In this article, we investigate security and privacy issues in networked supervisory control systems over multiple channel networks. We consider a networked discrete-event system controlled by a supervisor that receives information from sensors and sends control decisions to actuators via observation channels and control channels, respectively. The security problem is studied for the scenario where some of the communication channels are insecure in the sense there exists a passive intruder (eavesdropper) that can access the information-flow in those insecure communication channels. We adopt the concept of opacity, an information flow security property, to characterize the security status of the supervisory control system. Specifically, we assume that the system has a secret and the system is said to be opaque if the intruder can never determine the secret of the system unambiguously based on the information-flow in the insecure channels. We consider the notions of current-state opacity, K-step opacity, and infinite-step opacity in the networked control setting. New network observers are proposed to estimate the state of the system with two-side incomparable channel information. We show that the opacity verification problems for the networked setting can be effectively solved using the proposed network observers.

Application of networked discrete event system theory on intelligent transportation systems

The responses of vehicles to the changes in traffic situations inevitably have delays in observing an event and implementing a control command, which often causes fatal accidents. So far, the methods for handling delays are empirical and cannot be mathematically proven. To eliminate the accidents caused by such delays, in this paper, we develop mathematically provable methods to handle these delays. Specifically, we use networked discrete event systems to model the process of driving vehicles and present a supervisory controller for handling delay situations. The method developed in this paper could serve as a new start for modeling and controlling the responsive behaviors of self-driving vehicles in the future.

Nonblocking and deterministic decentralized control for networked discrete event systems under communication delays

Nonblocking is an important issue in supervisory control of discrete event systems. In this paper, we investigate nonblocking control for networked discrete event systems. In networked discrete event systems, there are two types of supervisory controls: nondeterministic control and deterministic control. Following the deterministic control, we investigate the nonblocking control problem in a decentralized control framework where communication among the plant and the local supervisors are subject to nondeterministic but bounded delays. We introduce delay co-observability to capture the nondeterminism caused by observation delays. With delay co-observability, we derive a necessary and sufficient condition for the existence of solutions of the nonblocking control problem. When the condition is satisfied, we propose to use state-estimate-based predictive local networked supervisors to control the plant. To illustrate the applications of the results, we consider a black start problem in micro grids.

Opacity of networked discrete event systems

Opacity is an important information-flow property, which is used to characterize whether or not the secrets of system have been leaked to the intruders. Over the past ten years, opacity of discrete event systems (DESs) modeled as automata and Petri nets have been well investigated under the assumption that the communication channels between systems and intruders are perfect. However, in networked discrete event systems (NDESs), the DESs run in a shared network, and the communication delays and losses are inevitable usually. In this paper, we investigate the problems of opacity in the framework of NDESs. We formulate four kinds of opacity for NDESs, including current-state opacity, initial-state opacity, initial-final-state opacity, and K-step opacity. We also present the verification algorithms for these notions. We extend the notions of opacity in classical DESs, that is, if the communication delays and losses disappear, then these new notions would reduce to their classical counterparts. A number of examples are provided to illustrate the results obtained.

Initial-State Estimation of Multi-Channel Networked Discrete Event Systems

Initial-state estimation consists in determining the initial state of the system based on the observation from the system and the system structure. It is the basis for many problems in security applications, such as opacity, detectability and supervisory control. However, when it comes to cyber-physical systems (CPS) communication delays and losses are inevitable. In this letter, we study the initial-state estimation problem in a CPS that is modeled with a multi-channel networked discrete event system (NDES). In practice, the system is usually distributed over several locations, and the communication between the system and an agent (e.g., intruder, observer, supervisor) is carried out via a shared network, in which communication delays and losses may happen. In multi-channel NDESs, even when the first-in first-out rule is satisfied in each channel, the order of events received by the agent may still be shuffled due to random communication delays in each channel. To address these new challenges, we formalize the initial-state estimation problem in multi-channel NDESs and a networked initial-state estimator is proposed to generate the exact initial-state estimate.

Information control in networked discrete event systems and its application to battery management systems

Opacity is an important property in control of information flow among networked agents. In this paper, we investigate information control problems in networked discrete event systems using opacity. In a networked discrete event system, communication among agents is via a shared communication network. Since delays and losses are unavoidable in networked discrete event systems, they must be considered in investigating opacity. We call opacity under communication delays and losses network opacity. We first define three network opacities: strong network opacity, weak network opacity, and network non-opacity. We derive necessary and sufficient conditions for network opacities and develop methods to check network opacities. We then apply network opacity to solve a problem in battery management systems.

Cooperative control of a flexible manufacturing system

Abstract This paper considers networked discrete-event systems. Local state-feedback controllers enable each subsystem to reach local target states. Due to the physical restrictions between the subsystems, the local target states might not be reachable autonomously, but cooperatively with the help of other subsystems. Therefore, each subsystem is extended by a network unit, which detects and resolves possible physical restrictions. If cooperation is necessary, the network units temporarily modify their local target states while applying situation-dependent communication. The proposed method is applied to the flexible manufacturing system HANS and its applicability to real-life systems is demonstrated by experimental results.

Fault-tolerant cooperative control of networked discrete-event systems

Abstract Networked discrete-event systems like smart manufacturing systems consist of subsystems, which are physically coupled and digitally connected by a communication network. The subsystems are able to solve their local tasks autonomously and to cooperate with another subsystem if necessary. In the second situation, the network units of both subsystems initiate the cooperation via the communication network. The flexibility of the subsystems to solve autonomous and cooperative tasks demonstrates an important property that is required for manufacturing systems in Industry 4.0. The paper concerns the situation that a fault occurs within a subsystem, which has to be detected and the controller has to be adapted. To this aim, the subsystems are extended by a diagnostic unit and a reconfiguration unit. The diagnostic unit detects and identifies the fault, whereas the reconfiguration unit applies a locally reconfigured controller. It is shown that if the fault has the property to be networked reconfigurable, the fault is locally reconfigurable without affecting the overall system.

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.

Revisiting strong detectability of networked discrete-event systems

Abstract In this paper, we propose a new notion of strong networked detectability of networked discrete-event systems (DESs) modeled by finite-state automata (FSAs) with respect to a first-in, first-out channel with communication losses and bounded transmission delays. We use a new method to obtain a polynomial-time algorithm for verifying strong networked detectability. Moreover, we study the relationship between the new notion and its counterpart studied by [Sasi and Lin 2018]. We prove that strong networked detectability is strictly weaker than its counterpart in [Sasi and Lin 2018] and also obtain a polynomial-time verification algorithm for its counterpart without any assumption, which is more efficient than the exponential-time algorithm given by [Sasi and Lin 2018] under several assumptions. Particularly, for FSAs with respect to an ideal channel, under two fundamental assumptions of deadlock-freeness and having no unobservable reachable cycle, we show that strong networked detectability is equivalent to its counterpart studied by [Sasi and Lin 2018].

State Estimation of Multichannel Networked Discrete Event Systems

In a networked discrete event system, communication between a plant and a supervisor/agent is carried out via a wired/wireless communication network, in which communication delays are unavoidable. Hence, how to obtain state estimates under communication delays is an important problem. The existing methods assume that there is only one communication channel and, thus, the first-in, first-out (FIFO) property is satisfied. In complex systems distributed over several locations, several channels are often used to send the event occurrences to the (central) supervisor/agent, in which the FIFO property holds only for each channel and communication delays may be different for different channels. To address these new challenges, this paper investigates the state estimation problem in multichannel networked discrete event systems. Two methods are proposed. The first method is relatively simpler and computes only conservative estimates. The second method is more complex, but it computes the exact state estimates. Examples are given to illustrate the results.

Online model composition for networked discrete-event systems

Networked discrete-event systems consist of physically coupled subsystems that utilise a digital communication network for solving their current tasks. Each subsystem has to be steered from its current state into a situation-dependent target state. Due to the physical couplings among the subsystems, certain state transitions have to be executed synchronously with other subsystems to reach the local target states. To satisfy all local tasks, the subsystems have to find a deadlock-free order of the synchronous state transitions by applying global model information. However, in the networked discrete-event system there does not exist any coordinator and the global model information is distributed among the subsystems. Hence, the subsystems have to exchange model information and to compose it online to find the deadlock-free order of synchronous state transitions. This paper proposes an online model composition algorithm based on the A⁎-Algorithm that determines the necessary local model information to be communicated and composed by the subsystems. The method is demonstrated by an example.

Detectability of networked discrete event systems

Detectability of discrete event systems, defined as the ability to determine the current and subsequent states, is very important in diagnosis, control, and many other applications. So far only detectability of non-networked discrete event systems has been defined and investigated. Non-networked discrete event systems assume that all the communications are reliable and instantaneous without any delays or losses. This assumption is often violated in networked systems. In this paper, we study detectability for networked discrete event systems. We investigate the impact of communication delays and losses on detectability. We define two classes of detectabilities: network detectability for determining the state of a networked discrete event systems and network D-detectability for distinguishing certain pairs of states of the systems. Necessary and sufficient conditions for network detectability and network D-detectability are derived. Methods to check network detectability and network D-detectability are also developed. Examples are given to illustrate the results.

Codiagnosability of networked discrete event systems subject to communication delays and intermittent loss of observation

Failure diagnosis is a crucial task in modern industrial systems, and several works in the literature address this problem by modeling the system as a Discrete-Event System (DES). Most of them assume perfect communication between sensors and diagnosers, i.e., no loss of observation of events, or event communication delays between the measurement sites and the diagnosers. However, industrial systems can be large and physically distributed, in which cases, communication networks are used to provide an efficient way to establish communication between devices. In diagnosis systems, the use of networks can introduce delays in the communication of event occurrences from measurement sites to the local diagnosers, leading to an incorrect observation of the order of occurrence of events generated by the system and, as a consequence, to an incorrect diagnosis decision by the local diagnoser. In this paper, we address the problem of decentralized diagnosis of networked Discrete-Event Systems subject to event communication delays, and we introduce the definition of network codiagnosability of the language generated by a DES subject to both event communication delays and intermittent loss of observation, and present necessary and sufficient conditions for a language to be network codiagnosable, for short. We also propose an algorithm to verify this property.

A new approach to tracking control of networked discrete-event systems

This paper presents a method for the tracking control of discrete-event systems that utilise digital communication only if the communication among the subsystems is necessary for solving the current task. The controlled subsystems are modelled by Input/Output (I/O)-automata and their local control aim is to reach given final states autonomously. The necessity of communication arises when there are synchronous state transitions on the state sequence into the final state, which may only be executed simultaneously by a set of subsystems. A new network unit is introduced, which extends each controlled subsystem in order to satisfy these cooperative tasks. It detects, initiates and ensures the simultaneous execution of the synchronous state transitions by means of the digital communication network. It is proved that the networked system is guaranteed to satisfy the local tasks in the networked discrete-event system, while executing the synchronous state transitions only synchronously. A cooperative car assembling process is used to demonstrate the proposed method.

Supervisory Control of Networked Timed Discrete Event Systems and Its Applications to Power Distribution Networks

This paper treats the problem of existence of a controller for networked-timed discrete-event systems. The results are new and useful for any networked discrete-event system for which it is important that time be explicitly considered in the model. We assume that a controller (supervisor) communicates with the system to be controlled via a shared communication network. Delays and losses in communication systems and their impacts on control are investigated. Under the assumption that the delays and losses are bounded, we derive a necessary and sufficient condition for the existence of a controller. The condition is expressed as network T-controllability and network T-observability, which are defined in this paper. We apply the results to power distribution network management, and illustrate the application using a 33-node (bus) test system, where the objective is to ensure that the total substation transformer power stays within prespecified safety limits.

Supervisor Synthesis for Networked Discrete Event Systems With Communication Delays

We investigate supervisory control of networked discrete event systems with asynchronous communication between the plant and supervisor. The observation and control may be randomly delayed. The delays are bounded and will not change the order of observation (FIFO). Our goal is to synthesize a supervisor such that the behavior of the supervised system is both adequate and legal (safe). We derive a necessary and sufficient condition for the existence of such a supervisor. When the condition is satisfied, we find a minimally-permissive state-estimate-based control policy which can be implemented offline or online. Furthermore, we derive an algorithm to check the condition for the existence of the supervisor by constructing an augmented automaton. The algorithm allows us to obtain a maximally-permissive control policy by iteration.

Decentralized control of networked discrete event systems with communication delays

In many practical systems, supervisory control is not performed by one centralized supervisor, but by multiple local supervisors. When communication networks are used in such a system as the medium of information transmission, the communication channels between local supervisors and the system to be controlled will unavoidably result in communication delays. This paper investigates how to use these local supervisors to control the system in order to satisfy given specifications even under communication delays. The specifications are described by two languages: a minimal required language which specifies the minimal required performance that the supervised system must have and a maximal admissible language which specifies the maximal boundary that the supervised system must be in. The results show that if the control problem is solvable, then there exists the minimal control policy which can be calculated based on state estimates. Furthermore, we derive algorithms to check whether the control problem is solvable or not.

Control of Networked Discrete Event Systems: Dealing with Communication Delays and Losses

In this paper, we investigate supervisory control of networked discrete event systems. Such systems are now widely used in practice. The key feature of a networked discrete event system is communication delays and losses in observation and control. Without addressing communication delays and losses in networked systems, no control will be effective. We propose to use some special observation mappings to deal with observation delays and losses. Based on these mappings, we define network observability. We also consider control delays and losses in networked discrete event systems and define network controllability. We show that these definitions capture precisely the requirements on effective supervisory control in networked discrete event systems in the sense that a networked supervisor exists if and only if both network controllability and network observability are satisfied.