Dear Mehdi,
I’m not sure what the problem is with Echo set to True, and I can’t reproduce the error without all of your input files.
Enabling the generator (GenDOF) and first tower fore-aft (TwFADOF1) degrees-of-freedom (DOFs) will result in a 4-state model with the states you want, plus the azimuth state. The azimuth state is often removed from a linearized model through post-processing, as discussed in the following forum topic: Fast Linearized Models (starting Sep 16, 2014).
Normally with TrimCase = 3, you want to set constant generator torque for the operating-point determination as explained in the Linearization chapter of the FAST User’s Guide. You can do this by setting the following input parameters in the FAST primary input file:
VSContrl = 1
VS_RtGnSp = 9999.9E-9
VS_RtTq = the desired constant torque; for the NREL 5-MW turbine, this is likely the rated generator torque, 43093.55 N-m
VS_Rgn2K = 9999.9E-9
VS_SlPc = 9999.9E-9
The 3rd bullet in my previous post explains how to add the blade-pitch angle and generator torque as linearized model inputs. To include the rotation of the rotor and fore-aft movement of the tower in the linearized model outputs, replace your OutList with only the following: “RotSpeed, TTDspFA, and NcIMUTVxs”. The first two items should be clear; for an explanation of NcIMUTVxs, see the following forum topic: Tower-top / yaw bearing inertia translational velocity.
Best regards,