Hierarchical robust supervisory control of discrete-event systems

The problem of Robust Supervisory Control (RSC) of Discrete-Event Systems (DES) is concerned with situations in which the DES plant model has dynamics uncertainty. A main challenge in the development of solutions for supervisory control problems (including RSC) is the issue of complexity of resulting solutions. Hierarchical approaches to supervision have been found to be effective in mitigating the above issue. In hierarchical control, a high-level supervisor designed based on a simplified high-level model of the plant, receives information about important events in the plant and issues high-level supervisory commands. In this thesis, the problem of hierarchical robust supervisory control under partial observation is studied. First, the setup of Zhong-Wonham for hierarchical control is extended to the case of control under partial observation. A Factorization property is derived that the reporting map must satisfy so that the reports sent to the high-level supervisor rely only on the low-level observable sequences. Furthermore, the three properties of Unobservable-and-Unique-Controllability (UUC), Unobservable-and-Uncontrollable-Prefixes-for-Observability (UUPO) and Partially-Observable-Strict-Output-Control-Consistency (PO-SOCC) are introduced and showed to ensure hierarchical consistency. Algorithms for modification of the plant model and reporting map (if necessary) to satisfy the Factorization, UUC, UUPO and PO-SOCC properties have also been developed. Next, the problem of robust supervisory control of a finite family of discrete-event plants is studied. Each plant has a separate closed specification language. A hierarchical solution is developed assuming full observation and then extended to the case of partial observation, following the approach in the thesis for hierarchical control under partial observation. Finally, a case study involving a flexible manufacturing system production line is studied where a machine is prone to failure. Following the approach developed in this thesis, a hierarchical robust supervisory control is designed to solve the control problem

Hierarchical interface-based supervisory control using the conflict preorder

Hierarchical Interface-Based Supervisory Control decomposes a large discrete event system into subsystems linked to each other by interfaces, facilitating the design of complex systems and the re-use of components. By ensuring that each subsystem satisfies its interface consistency conditions locally, it can be ensured that the complete system is controllable and nonblocking. The interface consistency conditions proposed in this paper are based on the conflict preorder, providing increased flexibility over previous approaches. The framework requires only a small number of interface consistency conditions, and allows for the design of multi-level hierarchies that are provably controllable and nonblocking

Supervisory Control of Product and Hierarchical Discrete Event Systems

International audienceIn this paper, the supervisory control of a class of Discrete Event Systems is investigated. Discrete event systems are modeled either by a collection of Finite State Machines that behave asynchronously or by a Hierarchical Finite State Machine. The basic problem of interest is to ensure the invariance of a set of particular configurations in the system. When the system is modeled as asynchronous FSMs, we provide algorithms that, based on a particular decomposition of the set of forbidden configurations, solve the control problem locally (i.e. on each component without computing the whole system) and produce a global supervisor ensuring the desired property. We then provide sufficient conditions under which the obtained controlled system is non-blocking. This kind of objectives may be useful to perform dynamic interactions between different parts of a system. Finally, we apply these results to the case of Hierarchical Finite State Machine

Hierarchical hybrid control: A case study

A case study of the difficulties encountered in the design of hierarchical, hybrid control systems is presented. As our example we use the Intelligent Vehicle Highway System (IVHS) architecture proposed for vehicle platooning, a system that involves both continuous state and discrete event controllers. We point out that even though conventional analysis tools suggest that the proposed design should fulfill certain performance requirements, simulation results show that it does not. We consider this as an indication that the conventional tools currently in use for the design and verification of control systems may be inadequate for the design of hierarchical controllers for hybrid systems. The analysis also indicates certain shortcomings of the current IVHS design. We propose solutions to fix these problems

Robust State-Based Supervisory Control of Hierarchical Discrete-Event Systems

Model uncertainty due to unknown dynamics or changes (such as faults) must be addressed in supervisory control design. Robust supervisory control, one of the approaches to handle model uncertainty, provides a solution (i.e., supervisor) that simultaneously satisfies the design objectives of all possible known plant models. Complexity has always been a challenging issue in the supervisory control of discrete-event systems, and different methods have been proposed to mitigate it. The proposed methods aim to handle complexity either through a structured solution (e.g. decentralized supervision) or by taking advantage of computationally efficient structured models for plants (e.g., hierarchical models). One of the proposed hierarchical plant model formalisms is State-Tree-Structure (STS), which has been successfully used in supervisor design for systems containing up to 10^20 states. In this thesis, a robust supervisory control framework is developed for systems modeled by STS. First, a robust nonblocking supervisory control problem is formulated in which the plant model belongs to a finite set of automata models and design specifications are expressed in terms of state sets. A state-based approach to supervisor design is more convenient for implementation using symbolic calculation tools such as Binary Decision Diagrams (BDDs). In order to ensure that the set of solutions for robust control problem can be obtained from State Feedback Control (SFBC) laws and hence suitable for symbolic calculations, it is assumed, without loss of generality, that the plant models satisfy a mutual refinement assumption. In this thesis, a set of necessary and sufficient conditions is derived for the solvability of the robust control problem, and a procedure for finding the maximally permissive solution is obtained. Next, the robust state-based supervisory framework is extended to systems modeled by STS. A sufficient condition is provided under which the mutual refinement property can be verified without converting the hierarchical model of STS to a flat automaton model. As an illustrative example, the developed approach was successfully used to design a robust supervisor for a Flexible Manufacturing System (FMS) with a state set of order 10^8

Supervisory Control of Asynchronous and Hierarchical Finite State Machines

International audienceIn this paper, modular supervisory control of a class of Discrete Event Systems is investigated. Discrete event systems are modeled by a Hierarchical Finite State Machine. The basic problem of interest is to solve the State Avoidance Control Problem. We provide algorithms that, based on a particular decomposition of the set of forbidden configurations, locally solve the control problem (i.e. on each component without computing the whole system) and produce a global supervisor ensuring the desired property. This kind of objectives may be useful to perform dynamic interactions between different parts of a syste

Supervisory control of discrete-event systems with output : application to hybrid systems

In this thesis, the problem of supervisory control of Discrete-Event Systems (DES) with output is presented and discussed at length. In such systems, causal output functions are employed to assign each sequence of inputs with a corresponding sequence of outputs. When the specification of the desired behavior is given by a formal language over the output alphabet, necessary and sufficient conditions are derived for the existence of nonblocking input as well as nonblocking output supervisory controls. An algorithm is presented to extend the results of nonblocking input/output supervisory control from language-based framework into finite automata framework, making the proposed results applicable to large scale discrete-event systems. The idea of siblings is introduced to solve the problem of nondeterminism in discrete-event abstractions of hybrid systems, giving rise to the development of a theory for nonblocking supervisory control of hybrid systems. Our results enable one to apply classical supervisory control theory to design supervisors for DES approximations of hybrid systems, and to import many interesting concepts from classical theory such as modular and hierarchical control

Safety Control of Hierarchical Synchronous Discrete Event Systems: A State-Based Approach

International audienceIn this paper, we discuss the control of a particular class of Hierarchical Discrete Event Systems and the state avoidance control problem is considered. A methodology is provided that locally computes on each component of the system the set of bad states (these are the states that may lead to the forbidden states via an uncontrollable trajectory). This is performed without computing the whole system. At this point, the supervisor is evaluated on the fly w.r.t. the bad states and thus requires an on-line evaluation in order to determine the set of events that has to be disabled by control. It is performed in such a way that the global partial transition function does not need to be built