An approach to control system design is described, based on an extension of the multilevel control concept. In essence, the problem is divided into simpler subproblems: (a) The controlled process is decomposed into a number of subprocesses, each with its own control system operating on a local suboptimal performance criterion; (b) each subprocess controller is decomposed into a hierarchy of control functions which distribute the load and responsibility for satisfying the control objective. The primary effort of the paper is in the development of the control hierarchy as a useful concept in control system design and implementation. The hierarchy is described in the context of the multilevel approach; however, an important distinguishing feature is the association of levels with disturbance sets roughly classified according to the frequency dependency of their effects on the overall performance. Also associated with the hierarchy is an ordering with respect to time scale, complexity of computation, degree of uncertainty, and so on, which relate to important design considerations for the overall system.

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