Unstructured model uncertainty

Unstructured model uncertainty relates to unmodelled effects such as plant disturbances and are related to the nominal plant CmCv) as either additive uncertainty (s)... 

In robustness analysis, the unstructured uncertainty is often expressed as bounds on a jominal process model. For example, suppose that G is the process model used for control system design, and that G is the transfer function for the actual (but unknown) process. They can be related by an unstructured model uncertainty, A(5) ... 

Stein and Doyle [35] developed an expression to calculate ft for the Robust Performance Problem in the case where the plant is minimum phase and is controlled by an inverse-based decoupling controller. The modeled uncertainty is described by a complex unstructured input block with weighting function w, and performance requirement Wj measured by the closed-loop sensitivity function S. The decoupling controller K is based on the inverse of G in the form. (s) = G (s) (s), where k(s) is a scalar transfer function which makes K s) proper and gives a stable closed-loop system. Note that G s) is a linear stable system with stable inverse (i. e. G is minimum-phase).This compensator produces diagonal sensitivity and complementary sensitivity functions with identical diagonal elements, namely... 

The robustness of a control system can be analyzed theoretically if the degree of process variability (or model uncertainty) can be characterized. The uncertainty can be structured (for example, parameter variations) or unstructured (for example, variations in the transfer function or frequency response). In this section, we consider the important concept of robust stability. The related concept of robust performance is described elsewhere (Morari and Zafiriou, 1989 Skogestad and Postlethwaite, 2005). 

The control system must not only compensate the nonlinearities of the vehicle dynamics, but also the dynamics that were not modeled, or unstructured uncertainties, as well as external disturbances. These uncertainties include river currents, the actuator system hydrodynamics, navigation, and control subsystem delays. 

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