476 research outputs found
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
Distributed Nonblocking Supervisory Control of Timed Discrete-Event Systems with Communication Delays and Losses
This paper investigates the problem of distributed nonblocking supervisory
control for timed discrete-event systems (DESs). The distributed supervisors
communicate with each other over networks subject to nondeterministic
communication delays and losses. Given that the delays are counted by time,
techniques have been developed to model the dynamics of the communication
channels. By incorporating the dynamics of the communication channels into the
system model, we construct a communication automaton to model the interaction
process between the supervisors. Based on the communication automaton, we
define the observation mappings for the supervisors, which consider delays and
losses occurring in the communication channels. Then, we derive the necessary
and sufficient conditions for the existence of a set of supervisors for
distributed nonblocking supervisory control. These conditions are expressed as
network controllability, network joint observability, and system language
closure. Finally, an example of intelligent manufacturing is provided to show
the application of the proposed framework
Detection and Prevention of Cyber-Attacks in Networked Control Systems
This paper addresses the problem of detection and prevention of cyber attacks in discrete event systems where the supervisor communicates with the plant via network channels. Random control delays may occur in such networked systems, hence the control of the supervisor could be affected. Furthermore, there is an attacker targeting the vulnerable actuators. The attacker can corrupt the control input generated by the supervisor, and aims at driving the plant to unsafe states. We propose a new approach to model the closed-loop system subject to control delays and attacks. The notion of AE-safe controllability in the networked control system is defined: it describes the ability to prevent the plant from reaching unsafe states after attacks are detected. A method for testing AE-safe controllability is also presented. Copyright (C) 2020 The Authors
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
activations 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 random 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 minimize sensor activations in a transition-based domain while
ensuring a given specification condition is satisfied. A practical example is
provided to show the application of the developed sensor activation methods.
Finally, we briefly discuss how to extend the proposed framework to a
decentralized sensing architecture
The Cyber Physical Implementation of Cloud Manufactuirng Monitoring Systems
AbstractThe rise of the industrial internet has been envisaged as a key catalyst for creating the intelligent manufacturing plant of the future through enabling open data distribution for cloud manufacturing. The context supporting these systems has been defined by Service Oriented Architectures (SOA) that facilitate data resource and computational functions as services available on a network. SOA has been at the forefront EU research over the past decade and several industrially implemented SOA technologies exist on the manufacturing floor. However it is still unclear whether SOA can meet the multi-layered requirements present within state-of-the-art manufacturing Cyber Physical Systems (CPS). The focus of this research is to identify the capability of SOA to be implemented at different execution layers present in a manufacturing CPS. The state-of-the-art for manufacturing CPS is represented by the ISA-95 standard and is correlated with different temporal analysis scales, and manufacturing computational requirements. Manufacturing computational requirements are identified through a review of open and closed loop machine control orientations, and continuous and discrete control methods. Finally the Acquire Recognise Cluster (ARC) SOA for reconfigurable manufacturing process monitoring systems is reviewed, to provide a topological view of data flow within a field level manufacturing SOA
- …