Plant adaptive systems versus ordinary feedback systems

The limitations in classical feedback that might justify the more complex plant or process adaptive systems are studied. Some of the limitations cited in the adaptive literature apply only to the classical single-degree-of-freedom configuration and not to the classical two-degree-of-freedom structures. Model and conditional feedback configurations are not superior to ordinary two-degree-of-freedom configurations. Time invariant (classical) compensation is adequate for coping with the sensitivity and disturbance problem in lightly damped and drifting plant poles. Two significant limitations in ordinary linear feedback systems that may justify the adaptive approach are: a) Their susceptibility to feedback transducer noise when the plant by itself does not have the loop gain area required for the desired sensitivity properties of the system; b) The limited sensitivity reduction achievable in nonminimum phase and unstable plants. The first of these may be eased by a multiple-loop design and the second by a parallel plant design. However, it has not been shown how the adaptive systems overcome the limitations of ordinary feedback systems.