Modern aircraft are often designed following the More Electric philosophy, but in some cases, like in the primary flight control, the EHAs (Electro-Hydraulic Actuators) are still widely used. In the first part of the thesis, the different actuation systems are presented with their various features, followed by an analysis of the servo-valve types most commonly used in electro-hydraulic actuation systems. This first part concludes with analyzing different servo valve characteristics such as non-linearities, focusing on the friction phenomenon and the most critical models. Among the friction models, theoretical and numerical models are analyzed. The modeling of friction in a numerical environment is undoubtedly the most complex.