Dear Zarei,
As I said, pitch actuator dynamics are not intrinsically included in FAST.
Please see the following post on our forum for information on the FAST theory basis: Coupled blade modes in FAST. WBut while the structural nonlinearities can be clearly seen e.g. in the “unofficial FAST theory manual”, the aerodynamic nonlinearities are more difficult to write down because e.g. the blade element / momentum (BEM) theory depends on the solution to nonlinear algebraic equations.
Best regards,