Reference model modification for robust performance conservation of model reference adaptive controllers

This paper evaluates the robust performance of Model Reference Adaptive Control (MRAC), which is applied to a short period model with pitch-break nonlinearity. The objective of the augmenting adaptive control law is to improve the performance in presence of the pitch-break nonlinearity, while still providing robust performance w.r.t. other parametric plant uncertainties. In particular, it is shown that, if standard MRAC approaches are designed to solve certain problems, the adaptive augmentation can lead to a degradation in robustness for other uncertainties in the plant dynamics which were not considered a-priori. In order to maintain at least the same level of robust performance as the baseline controller, several modifications of the reference model are suggested. The suggested modifications adjust the reference model with the objective to allow a larger set of reference trajectories to exist. This is less restrictive, and therefore the reference trajectories remain achievable by the plant.