# pitch and torque spikes on variations

Hi!
So, I’ve been “living” with this issue for a while now, at first I simply accepted it and manage to reduce its effect but now I’d like to be sure if is creating some bad dynamics in my system.

What happens is that when I send a different reference to pitch and torque to FAST in SIMULINK it gets some spikes, imagem below will clarify this.
It is possible to reduce the spike by adding a filter prior to sending the value to FAST, but it will still be there, but mitigated.

I saw a close problem to this in a different topic and, as the below quote, tried the following solutions without success:

Dear Leanardo,

The FAST S-Function will use extrapolated values of the inputs from Simulink when advancing from time step n to n+1 (linear or quadratic extrapolation, depending on your setting of InterpOrder). This extrapolation may cause problems (e.g. the spikes you are seeing) if the pitch-angle command is not a smooth function e.g .when stepping up the pitch angle as you are. I would suggest ramping up the pitch angle with a smooth function to avoid problems with the extrapolation (preferably with a function whose first derivative is defined e.g. an S-shaped curve would be better than a step or a linear ramp between distinct pitch angles).

Best regards,

Thank you again for your accurate and quick response!

Indeed by adding an s-shaped smoother fixed the issue.

For anyone having the same problem:

Add a transfer function of the form “Y(s) = H(s)X(s) where H(s) = Kwo^2/(s^2 + 2zetawo*s + wo^2). Here you define zeta = 1.0. Then the s-shape is retained for any K and wo value. Note that X(s) has already been hit by a ramp. In matlab or any other tools, linear ramp and 2nd lag are standard blocks.”

All you gotta do is make that TF fast enough by increasing “wo”