Hybrid systems in process control

Modeling and control of hybrid systems, with particular emphasis on process control applications, are considered in this article. Based on a number of observations on typical mixed discrete and continuous features for such applications, a fairly general model structure for hybrid systems is proposed. This model structure, which clearly separates the open-loop plant from the closed-loop system, is suitable for analysis and synthesis of hybrid control systems. To illustrate this, three different approaches for control-law synthesis based on continuous and discrete specifications are discussed. In the first one, the hybrid plant model is replaced by a purely discrete event model, related to the continuous specification, and a supervisor is synthesized applying supervisory control theory suggested by Ramadge and Wonham (1987). The other two methods directly utilize the continuous specification for determination of a control event generator, where time-optimal aspects are introduced as an option in the last approach.