Supervisory control of distributed systems: Conflict resolution

In distributed synthesis and control, one well-known potential hazard is co nflict between modular designs. In a modular approach to the supervisory co ntrol of discrete-event systems, modular supervisors that are individually nonblocking (with respect to the plant) may nevertheless conflict and thus produce blocking, or even deadlock, when operating concurrently. A scheme o f resolving this potential conflict between the modular supervisors would b e to accord priorities to the conflicting supervisors. When conflict arises , the modular supervisor that is assigned a higher priority will have sole control, or in other words the control action of the lower priority supervi sor will be suspended. Thus by assigning priority appropriately, control ac tions of the modular supervisors will be suspended and reactivated in such a way that the potential conflict can be averted. In this article we formal ize this scheme with reporter maps from a hierarchical approach to the supe rvisory control of discrete-event systems. These maps, each acting as an in terface between a modular supervisor and the plant, mediate the flow of inf ormation and control, and thus in this way achieve suspension and reactivat ion of the modular supervisors. Sufficient conditions on these reporter map s for conflict resolution are obtained. Roughly speaking, the conditions ar e that (1) the reporter maps select suitable 'subsystems' of the plant; (2) within these subsystems, conflicts are 'resolved'; (3) the reporter maps a re 'refined' enough to lift these local conflict resolutions back to the or iginal plant. With these conditions, a constructive solution is developed, which in essence suspends a supervisor 'just in time' to prevent conflict a nd reactivates it when the plant and the other supervisor return to the sta te they were in when the suspension began. Examples inspired by the feature interaction problem in telecommunication systems are provided for illustra tion.