A Discrete-Event System (DES) is a dynamic system that evolves with the abrupt occurrences of physical events. Such systems generally encompass processes that are discrete in time and state space, often asynchronous, and typically nondeterministic. Generally, manufacturing systems can be modeled as discrete-event systems.

Because of the nondeterministic nature of behaviour of a manufacturing system, its supervisory control must be carried out in closed loop. These traits complicate and greatly increase the complexity of the supervisory-control implementation. Thus, control of even moderately complex systems can easily require an immensely large DES strategy.

A split approach that uses some alternate mechanism in addition to a DES supervisory controller might be able to control a moderately complex system without requiring an immense DES strategy. This mechanism would assert control whenever events diverged from the states of the DES supervisory controller. Thus, the objective of this work was the development of a supervisory controller for a manufacturing system that splits operations between a DES supervisory controller and some other mechanism(s). This modified approach to DES control provides a more efficient controller than could reasonably be attained using solely a DES supervisory controller.

The Hybrid Supervisory Controller (HSC) that was developed consisted of three main elements:​ (i) the DES supervisory controller which contains the nominal supervisory-control strategy, (ii) a diagnostic system, which monitors the workcell, identifies errors and initiates recoveries, and (iii) a mechanism, called the Alternate-Strategy Driver (ASD), that asserts control over the strategy of the supervisor, and which generates alternate part routes when needed.

The special requirements of the DES supervisory controller required its division into two modular supervisors. The first enforced safety specifications. The other enforced specifica. tions for the part routes, and is an aggregation of several modular sub-supervisors as well, one for each part that is being routed.

Testing via simulation showed that the HSC could:​ (i) re-route parts around failed equipment, (ii) actively resolve deadlocks, and (iii) successfully enable and disable operations within the workcell.