Forcible Events Research Articles

Supervisory Control Synthesis of Timed Automata Using Forcible Events

Supervisory Control Synthesis of Timed Automata Using Forcible Events

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.

On Controllability of Hybrid Systems

In this article, we investigate controllability of hybrid systems. We use hybrid machines to model such hybrid systems. The model consists of an automaton and a linear time-invariant system. The control goal is to drive the continuous state from any initial state to any final state. To achieve this goal, controls at both the discrete-event level and the continuous-variable level are used. Control at the discrete-event level can force some forcible events, and disable some controllable events. For the case that all events are controllable and forcible, and the graph of the automaton is strongly connected, a necessary and sufficient condition for controllability is obtained, using some existing results on controllability of switched linear systems. For three other cases, sufficient conditions are derived. If a hybrid system is not controllable, we design a supervisor to control the discrete-event part to ensure that the system becomes controllable after some finite transitions if possible. We find a necessary and sufficient condition for such a discrete-event control to exist.

Decentralized Supervisory Control of Timed Discrete Event Systems with Conditional Decisions for Enforcing Forcible Events

Decentralized Supervisory Control of Timed Discrete Event Systems with Conditional Decisions for Enforcing Forcible Events

Non-blocking Supervisory Control of Timed Automata using Forcible Events

Abstract Real-valued clocks make the state space of timed automata (TA) infinite. Conventional supervisory control synthesis techniques are only applicable to finite automata (FA). Therefore, to synthesize a supervisor for TA using conventional techniques, an abstraction of TA to FA is required. For many applications, the abstraction of real-time values results in an explosion in the finite state space. This paper presents a supervisory control synthesis technique applied directly to TA without any abstraction. The plant is given as a TA with a set of uncontrollable events and a set of forcible events that may preempt the passage of time. To obtain a non-blocking controlled system, a synthesis algorithm is proposed that delivers a supervisor (also as a TA) avoiding the blocking states. The algorithm works by (iteratively) strengthening the guards of edges labeled by controllable events and invariants of locations where time progress can be preempted by forcible events.

Maximally Permissive Supervisory Control of Timed Discrete-Event Systems under Partial Observation

Abstract In this paper, we investigate the supervisory control problem for timed discrete-event systems (TDES) under partial observation. In the timed setting, the system consists of both standard logical events and time event, where the former can be disabled directly by the supervisor if it is controllable while the latter can only be preempted by forcing the occurrences of forcible events. We consider a general control mechanism where the supervisor can choose which events to force dynamically online at each instant. The design objective is to synthesize a maximally-permissive supervisor to restrict the behavior of the system such that the closed-loop language is within a safe specification language. Effective procedure is presented to synthesize such a supervisor. To our knowledge, how to synthesize a maximally-permissive partial-observation supervisor has not been solved for timed DES. We provide a solution to this problem under a general control mechanism.

Supervisor Localization of Timed Discrete-Event Systems Under Partial Observation

We study supervisor localization for timed discrete-event systems under partial observation in the Brandin–Wonham framework. First, we employ timed relative observability to synthesize a partial-observation monolithic supervisor; the control actions of this supervisor include not only disabling action of prohibitible events (as that of controllable events in the untimed case) but also “clock-preempting” action of forcible events. Accordingly, we decompose the supervisor into a set of partial-observation local controllers one for each prohibitible event, as well as a set of partial-observation local preemptors one for each forcible event. We prove that these local controllers and preemptors collectively achieve the same controlled behavior as the partial-observation monolithic supervisor does. The above-mentioned results are illustrated by a timed workcell example.

The Effects of Non-Performance of Contract as a Result of Frustration of Purpose

Appearing of forcible events or other reasons which hinder the performance of contract or cause its frustration has special legal effects and destroy the casual link between the obligator and the loss resulting from its non-performance. In this situation, these reasons affect the structure of contract and possibly may cause the discharge or suspension of contract. Therefore, the effect of occurring this kind of excuse are twofold. One of the important consequences of occurring this event is the exemption of the obligator from contractual responsibility, because, the non-performance is not attributed to him. The other effect is that force majeure changes the structure of the contract. A contract that encounters to force majeure may be discharged forcibly or willingly or its performance suspended. Our purpose in this article is to review these effects.

Relative Observability and Coobservability of Timed Discrete-Event Systems

We study supervisory control of timed discrete-event systems (TDES) under partial observation, and propose new observability concepts effective for supervisor synthesis. First, we consider monolithic/centralized supervisory control, and introduce timed relative observability and timed relative weak observability . The former concept extends our previous work to the timed case, while the latter exploits choices of forcible events to preempt the clock event tick . We prove that timed relative (respectively, weak) observability is stronger than timed (respectively, weak) observability, weaker than normality, and closed under set union; hence there exists the supremal relatively (respectively, weakly) observable sublanguage of a given language.

On Relative Observability of Timed Discrete-Event Systems

We study new observability concepts in supervisory control of timed discrete-event systems (DES) under partial observation. Known concepts in the literature are timed observability and weak observability; neither, however, is closed under set union, and consequently the supremal (weak) observable sublanguage of a given language need not exist in general. In the first part of this paper, we extend our previous work on relative observability to timed DES: a fixed, ambient language is given, relative to which timed observability is tested. Relative observability is stronger than timed observability, weaker than normality, and closed under set union. An algorithm is presented for computing the supremal relatively observable sublanguage of a given language. In the second part, we identify relative weak observability by exploiting forcible events to preempt the clock event tick. Relative weak observability is stronger than weak observability, weaker than normality, and closed under set union; we present an algorithm for computing the supremal relatively weakly observable sublanguage of a given language.

Decentralized Supervisory Control of Timed Discrete Event Systems Using a Partition of the Forcible Event Set

Decentralized Supervisory Control of Timed Discrete Event Systems Using a Partition of the Forcible Event Set

Grid automata and supervisory control of dense real-time discrete event systems

We present a generalization of the classical supervisory control theory for discrete event systems to a setting of dense real-time systems modeled by Alur and Dill timed automata. The main problem involved is that in general the state space of a timed automaton is (uncountably) infinite. The solution is to reduce the dense time transition system to an appropriate finite discrete subautomaton, the grid automaton, which contains enough information to deal with the timed supervisory control problem (TSCP). The plant and the specifications region graphs are sampled for a granularity defined in a way that each state has an outgoing transition labeled with the same time amount. We redefine the controllability concept in the context of grid automata, and we provide necessary and sufficient solvability conditions under which the optimal solution to centralized supervisory control problems in timed discrete event systems under full observation can be obtained. The enhanced setting admits subsystem composition and the concept of forcible event. A simple example illustrates how the new method can be used to solve the TSCP.

Control-invariance of hybrid systems with forcible events

This paper considers controlled hybrid automata with forcible events. First, we introduce two semantics of the controlled hybrid automata using labeled transition systems. Next, we extend a concept of control-invariance in discrete event systems and show necessary and sufficient conditions for a predicate to be control-invariant. Finally, we show that there always exists the supremal control-invariant subpredicate for any predicate.

State Feedback Control in Hybrid Systems with Forcible Events

This paper shows necessary and sufficient conditions for the existence of state feedback in hybrid systems described by hybrid automata with forcible events. We introduce two semantics of controlled hybrid automata using transition systems. The conditions are based on the transition systems. It is also shown that the conditions are preserved under bisimilar relations so that synthesis methods of state feedback in discrete event systems can be applied in the hybrid systems if the bisimilar relations are finitary.

Supervisory control of timed discrete-event systems

The Ramadge-Wonham framework for control of discrete event systems is augmented with timing features by use of Ostroff's semantics for timed transition models. It is shown that the RW concept of controllability and the existence of maximally permissive supervisory controls can be suitably generalized. The enhanced setting admits subsystem composition and the concept of forcible event as an event that preempts the tick of a global clock. An example of a simple manufacturing cell illustrates how the new framework can be used to solve synthesis problems which may include logic-based, temporal and quantitative optimality specifications.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>