Within the discrete-event system setting, an abstract hierarchical control theory is developed on the basis of the concepts of control structures and observers. Control structure is an abstract generalization of the family of controllable sublanguages in the Ramadge-Wonham framework. With control structures, we establish a general version of Zhong's hierarchical consisten cy by first achieving control consistency - preservation of control structures through the aggregation mapping in a two-level hierarchy. Then we extend th e hierarchical theory to the setting of timed discrete-event systems. For a refinement of hierarchical consistency with preservation of the nonblocking property, the concept of observer is introduced via congruences on nondeterministic transition structures. Characterizations of observers with prefix-closure and postfix-closure operators are given. A close relation wa s discovered between our observer and Milner's observation equivalence.
On the basis of control structures and observers other architectural situati ons are studied, namely modular control, coordination, and decentralized control . Modular control is equivalent to the requirement that sets of controllable languages in a control structure be lattices; a hierarchical coordination sc heme is set up to resolve conflicts among modular supervisors; and conditions are obtained under which the concurrent behaviour of two decentralized superviso rs is the same as global supervision.
Computational and implementational aspects of observers, control structures,
and
control consistency are investigated.
