Tham khảo tài liệu 'quality management and six sigma part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Longitudinal Robust Stability Augmentation for Micro air Vehicle - Design and Validation 233 certain level of closed-loop performance is achieved for all plants in a specified set or for all variations in the plant parameters. On the other hand stability robustness is achieved if the closed loop remains stable in spite of modeling errors due to high frequency un-modeled dynamics and plant parameter variations. The following subsections explain the representation of the uncertainty and the conditions for robustness of the closed loop system in the face of the uncertainties. Representation of uncertainty Robust controller synthesis considers nominal model of the plant along with some assessment of its errors called model uncertainties. A key assumption in the mathematical models of uncertainty or errors is that the uncertain part of the dynamics can be modeled separately from the known part as represented by a nominal model. The mechanism used to represent the uncertainties is called representation of uncertainty. In robust control literature the mathematical representation of uncertainties caused by unintentional exclusion of high frequency dynamics generally take many forms of which the most commonly used is the multiplicative and additive uncertainty model. The additive and multiplicative uncertainties can be mathematically represented as Gp s Gp0 s Aa s 9 Gp s Gp0 s I Ai s 10 Gp s I Ao s Gp0 s 11 where Aa represents an additive perturbation Ai an input multiplicative perturbation and Ao an output multiplicative perturbation. Gp0 s is a nominal plant transfer function which is a best estimate in some sense of the true plant behavior and Gp s denote the true transfer function of the plant. In general uncertainties are represented in two forms. uncertain state space model which is relevant for systems described by dynamical equations with uncertain and or time-varying coefficients and linear fractional representation where the uncertain system is described .
