Robust decentralized supervisory control of discrete-event systems

In this thesis we study robust supervisory control of discrete event systems in two different settings. First, we consider the problem of synthesizing a set of decentralized supervisors when the precise model of the plant is not known, but it is known that it is among a finite set of plant models. To tackle this problem, we form the union of all possible behaviors and construct an appropriate specification, from the given set of specifications, and solve the conventional decentralized supervisory control associated with it. We also prove that the given robust problem has a solution if and only if this conventional decentralized supervisory control problem has a solution. In another setting, we investigate the problem of synthesizing a set of communicating supervisors in the presence of delay in communication channels, and call it Unbounded Communication Delay Robust Supervisory Control problem (UCDR-SC problem). In this problem, We assume that delay is unbounded but it is finite, meaning that any message sent from a local supervisor will be received by any other local supervisors after a finite but unknown delay. To solve this problem, we redefine the supervisory decision making rules, introduce a new language property called unbounded-communication-delay-robust (UCDR), and present a set of conditions on the specification of the problem. We also show that the new class of languages that is the solution to this problem has some interesting relations with other observational languages

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