State Tree Structures Research Articles

Nonblocking supervisory control of state-tree structures with event forcing

For complex discrete-event systems, state-tree structures (STSS) have demonstrated significant advantages in terms of the modeling and supervisor synthesis. The notorious state explosion problem can be effectively managed in STS. In this paper, the STS framework is enhanced with the concept of event forcing. Intuitively, event forcing is utilized to deny permission for the occurrence of undesirable competing events, that is, such events are directly or indirectly disabled. To reflect event forcing in STS, a systematic on-the-fly procedure during supervisor synthesis is proposed, which benefits the computational effort of computer memory and time cost. In addition, an incrementally iterative algorithm is developed and the computational complexity of it is polynomial with respect to the number of binary decision diagram nodes in use. Finally, several case studies are provided for evaluating the performance of the method. The experimental results show that the proposed method is computationally efficient for supervisor synthesis of a discrete-event system imposed event forcing.

Real-Time Scheduling Based on Nonblocking Supervisory Control of State-Tree Structures

This article presents a novel framework for the modeling and scheduling of real-time systems (RTS) processing both sporadic and (multiperiod) periodic tasks, based on nonblocking supervisory control of state-tree structures. Instead of assigning static priorities, a real-time scheduling mechanism, namely priority-free conditionally preemptive scheduling, is used to describe the preemption relation among the tasks processed in an RTS. As a dynamic priority scheduling mechanism, partially preemptive or nonpreemptive earliest-deadline first scheduling is also addressed in this article. Finally, the scheduling strategies are illustrated by real-world examples.

Top-Down Nested Supervisory Control of State-Tree Structures Based on State Aggregations

With a structured state space, state-tree structures (STS) are a powerful framework to model hierarchical finite state machines (HFSM). The boundary consistency property of STS endows them a compact and neatly representation. In this study, by naturally decomposing an STS into a set of STS nests in a top-down nested approach and finding a supervisor for each, the boundary consistency property is extended to the supervisory control of STS. As a consequence, the state spaces for both the system model and optimal supervisor are significantly reduced. Two examples are provided, in which the state space of a large scale HFSM example is reduced from 1024 to 2 × 1018.

Failsafe Mechanism Design for Autonomous Aerial Refueling using State Tree Structures

Autonomous Aerial Refueling (AAR) is vulnerable to various failures and involves cooperation among autonomous receivers, tankers and remote pilots. Dangerous flight maneuvers may be executed when unexpected failures or command conflicts happen. To solve this problem, a failsafe mechanism based on State Tree Structures (STS) is proposed. The failsafe mechanism is a control logic that guides what subsequent actions the autonomous receiver should take, by observing real-time information of internal low-level subsystems such as guidance and drogue&probe and external instructions from tankers and pilots. To generate such a controller using STS, the AAR procedure is decomposed into several modes, and safety issues related with seven low-level subsystems are summarized. Then common functional demands and safety requirements are textually described. On this basis, the AAR plants and specifications are modeled by STS, and a supervisor is synthesized to control the AAR model. To prove its feasibility and correctness, a simulation environment incorporating such a logic supervisor is built and tested. The design procedures presented in this paper can be used in decision-making strategies for similar flight tasks. Supporting materials can be downloaded in Github, [ https://github.com/KevinDong0810/Failsafe-Design-for-AAR-using-STS ] including related software, input documents and output files.

Nonblocking Supervisory Control of State-Tree Structures With Conditional-Preemption Matrices

State-tree structure (STS) is a powerful tool for the modeling and control of large-scale discrete-event systems (DES) whose structure is organized in a top–down hierarchy. The notorious state-explosion problem can be managed effectively in the framework of STS. Priority is an important concept and exists in a wide range of DES, such as manufacturing systems, traffic systems, and logistic (service) systems. In this article, a conditional-preemption matrix is used to describe the preemption relations among events, which can represent the priority intuitively. In order to augment STS with priority, we propose a novel STS framework: STS with conditional-preemption matrices (STSM), which aims to solve different kinds of issues related to priority. The optimal nonblocking supervisory control is deployed in the framework of STSM. Symbolic synthesis algorithms are implemented and integrated in a software tool, STSLib, which exploits binary decision diagrams as a basis for efficient computation. The developed approach is demonstrated by (a) manufacturing systems using automatic guided vehicles and (b) real-time systems.

Synthesis of Supervisory Control With Partial Observation on Normal State-Tree Structures

Supervisory control theory (SCT) of a discrete-event system (DES) is well developed to find its maximally permissive supervisor. As an extension of SCT, a new framework, state-tree structures (STS), has been deployed to manage the state explosion problem of SCT. The supervisory control with partial observation is investigated in SCT, of which the state explosion issue remains to be explored. This paper dwells upon an approach to the synthesis of supervisory control with partial observation in the normal STS framework, requiring that the conjunction of any two projected subpredicates should be false. First, following the definition of the observability based on STS, it is proven that there does not exist the supremal observable subpredicate. Second, we construct a subclass of observable subpredicates of a given predicate (obtained as a specification) on the basis of normal STS. Third, a largest element is proven to exist in the subclass, and thus a weakly controllable, coreachable, and observable subpredicate is computed to solve the supervisory control problem of the normal STS by an iterative algorithm. An illustrative example with state size over 109 is given to show that the proposed algorithm based on STS is superior to the approach based on SCT, which leads to the program crashes in SCT. Note to Practitioners —DES is a collection of many contemporary computer-integrated industrial and social service systems. For the sake of technology or cost considerations, the occurrences of some events are not fully observed, rendering the supervisory control algorithmically complicated and exacerbating the originally notorious state explosion problem. This paper aims to ameliorate the state explosion problem using a novel formalism, namely, state-tree structures, that can be applied to real-world systems thanks to the computational efficiency that is demonstrated by an industrial system. Compared with the traditional SCT of DES, the significance of the proposed methodology will intrigue academic researchers and industrial practitioners.

Supervisory control of state-tree structures with partial observation

Supervisory control of discrete-event systems (SCDES) is well developed to find a maximally permissive supervisor. As an extension to supervisory control theory, a new framework, state-tree structures (STS), has been deployed to manage the state explosion problem of SCDES. This paper aims to address this notorious issue of supervisory control with partial observation in the STS framework by state feedback control that calculates the controllers of the controllable-observable events only, which is realized by the following two steps. First, for a specification represented as a predicate, a supremal normal subpredicate that requires only the controllable-observable events enabled/disabled, is computed. Second, according to the new transition function constructed by the natural projection of the given STS, the supremal nonblocking, weakly controllable subpredicate is obtained from the supremal normal subpredicate. The proposed approach based on STS provides the possibility to supervise controllable events under partial observation in large-scale systems with the state explosion problem managed. An example with state size over 107 that leads to program crashes in SCDES can be solved in this paper. Moreover, in order to demonstrate the industrial applications of the contribution of this research, three examples are addressed.

State-Based Control of Discrete-Event Systems Under Partial Observation

This paper describes a state-based approach for supervisor synthesis of discrete-event systems under partial observation, based on predicates and predicate transformers. We focus on the normality property and provide an iterative algorithm for state-based normality synthesis. A condition is provided to simplify the algorithm. To bridge the gap between language-based normality synthesis and state-based normality synthesis, we prove that their synthesis results are mutually consistent. This paper also aims to build a useful foundation for non-blocking supervisor synthesis of partially-observed state tree structures, which is essentially a state-based approach. The proposed approach is illustrated with a Guideway example taken from the literature.

Exploiting symmetry of state tree structures for discrete-event systems with parallel components

ABSTRACTWe consider discrete-event systems consisting of parallel arrays of machines and buffers. The machines are divided into groups in each of which the members have identical structure, i.e. same state set and isomorphic transitions. This feature allows event relabelling of the machines in a given group to a standard prototype machine. In these systems, to avoid the underflow or overflow of the buffers, the controller needs only the information of the total numbers of components at each state and the numbers of workpieces in the buffers. By exploiting the identical structure of each group, we extract such control information from the control functions computed by the state tree structures to generate abstract control functions. Thanks to the symmetry of the system, we show that all controllable events relabelled to the same symbol share an invariant abstract control function, which is independent of the total number of machines, as long as the buffer sizes are fixed. The approach is illustrated by two examples.

Supervisor Localization of Discrete-Event Systems Based on State Tree Structures

Recently we developed supervisor localization, a top-down approach to distributed control of discrete-event systems in the Ramadge-Wonham supervisory control framework. Its essence is the decomposition of monolithic (global) control action into local control strategies for individual agents. In this technical note, we establish a counterpart localization theory in the framework of State Tree Structures, known to be efficient for control design of very large systems. We prove that the collective localized control behavior is identical to the monolithic optimal (i.e. maximally permissive) and nonblocking controlled behavior. Further, we propose a new and more efficient localization algorithm which exploits BDD computation.

Modular Supervisory Control of Discrete-event Systems Based on State Tree Structures

Modular supervisory control of discrete-event systems based on state tree structures(STS)is studied.The plant is modeled as a state tree structure and the specification is given as a predicate.Assuming this specification can be decomposed into the conjunction of subspecifications,we synthesize optimal nonblocking sub-supervisor for each subspecification.The behavior of the plant under the control of these sub-supervisors meets the requirements of the specification and is nonblocking and optimal.We determine conditions under which it is possible to synthesize the optimal control in a modular fashion.When the closed-loop system is blocking,we introduce a coordinator to solve it and give the method of synthesizing the coordinator.

Modular supervisory control and coordination of state tree structures

Optimal nonblocking modular supervisory control of discrete-event systems is developed using state tree structures to manage state explosion. The total specification of the system to be controlled is decomposed into several sub-specifications, and a separate optimal (maximally permissive) nonblocking supervisor designed for each. Under an additional global nonblocking condition we directly obtain an optimal nonblocking modular state feedback control for the full system. If that condition fails, i.e. the modular controlled system is blocking, an additional coordinator is adjoined which renders the global controlled behaviour, both nonblocking and optimal.

Nonblocking Supervisory Control of State Tree Structures

It is well known that the nonblocking supervisory control problem is NP-hard, subject in particular to state space explosion that is exponential in the number of system components. In this paper we propose to manage complexity by organizing the system as a state tree structure (STS). STS are an adaptation of statecharts to supervisory control theory. Based on STS we present an efficient recursive symbolic algorithm that can perform nonblocking supervisory control design (in reasonable time and memory) for systems of state size 10/sup 24/ and higher. The resulting controllers are tractable and readily comprehensible.