Different Pitch rate in and outside FAST SFunction in Simulink

[Yunpeng.Song]

Dear Dr. Jonkman,

I meet some problem when I’m trying to modling a modified pitch control loop in Simulink.
In my pitch control loop, the pitch rate been limited between -8deg/s to +8deg/s, however the output signal show very large fluctuation and the pitch rate in output signal is largly over my limitation. (you can find the time series of pitch angle & pitch rate in attachment.)

May I know why my opration signal been changed in FAST, is there a mechanism in the FAST to modify the received control signals?

Thanks

Best Regards,
SONG

[Jason.Jonkman]

Dear Yunpeng,

Similar questions have been asked and answered before, e.g.:

Pitch control error

Dear Barry, The rapidly varying blade-pitch angles you are seeing in your Simulink model are likely caused by the extrapolation of inputs implemented within the FAST S-Function (as discussed in other forum topics e.g. S-Function input output signal mismatch - #2 by Jason.Jonkman). My guess is the pitch angle of 5 degrees that you are setting in your Simulink model does not match the initial blade-pitch angle set within the ElastoDyn primary input file, hence the large variation in pitch angles…

pitch and torque spikes on variations

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 (prefer…

S-Function input output signal mismatch

Dear Joannes, The FAST v8 S-Function for the MATLAB/Simulink interface is a discrete-time block, meaning that inputs go in at time step “n” and outputs come out at time step “n+1”. Within the block, the output calculation at n+1 involves an extrapolation of inputs (linear or quadratic), which is normally negligible when the inputs vary slowly in time, but is visible when the inputs vary a lot between time steps. The error seems to imply that the model has going unstable, with vary large unrea…

Start-up of NREL 5MW

Dear Jason, Thanks for your immediate reply. Yes, you are right, it seems to be related to the initial conditions. I thought about that before, but just wasn’t aware how sensitive the controller is on poor initial conditions. Animated by your post, I run a simulation with TPCOn = 0, a steady wind of 12 m/s and an initial rotor speed of 12.1 rpm - and everything went fine. However, even with a slight change of the initial rotor speed to e.g. 12.25 rpm, the pitch controller becomes unstable resu…

Best regards,

[Yunpeng.Song]

@Jason.Jonkman

Dear Yunpeng,

Similar questions have been asked and answered before, e.g.:
Pitch control error - #17 by Jason.Jonkman
pitch and torque spikes on variations - #2 by Jason.Jonkman
S-Function input output signal mismatch - #2 by Jason.Jonkman
Start-up of NREL 5MW - #3 by Paul.Schunemann

Best regards,

Thank for your kind reply. These topics realy helped me a lot.

Best regards,
SONG