Pitch control error

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Barry.Dolan
Posts: 6
Joined: Mon Oct 24, 2011 5:23 am
Organization: Solutions 4 Energy
Location: Germany

Re: Pitch control error

Postby Barry.Dolan » Mon Dec 10, 2018 7:56 am

Hello,

I'm having a similar problem with inputs from Simulink. I feel that the topic wasn't completely resolved in the previous posts. So I hope to clarify the issue with my post.

I'm using FAST v8.16 on a 64bit Windows 7 installation with Matlab R2017a.

I'm trying to run Test18.fst with OpenLoop.mdl with the following modifications:
PCMode: 4
ServoData/Discon_x64.dll

The simulation completes successfully with a constant blade pitch angle of 0°.
However, when I specify a constant non-zero blade pitch angle of e.g. 5° the simulation crashes with the following error in the matlab command window:

Error using Run_OpenLoop (line 32)
Error reported by S-function 'FAST_SFunc' in 'OpenLoop/FAST Nonlinear Wind Turbine/S-Function':
FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates(node 8, blade 2):BEMT_UnCoupledSolve:DeterminePhiBounds:There is no valid value of phi for these operating
conditions! Vx = 0.99766, Vy = -21.502, rlocal = 24.145, theta = 0.1716

From the output file it is evident that only 4 time steps are computed and the blade pitch angle oscillates wildly (from 0° to +45° and back to -45°). This is obviously what causes the error displayed in the matlab command window.

I don't think I've made any modelling mistakes as all I've done is modify the model to accept its pitch angle input from Simulink and then supply it with a constant non-zero pitch angle.

Could the FAST_SFunc be incompatible with my version of Matlab?

Best regards,
Barry.

Jason.Jonkman
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Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
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Re: Pitch control error

Postby Jason.Jonkman » Mon Dec 10, 2018 8:16 am

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. viewtopic.php?f=4&t=1985&p=10496 and viewtopic.php?f=30&t=1720&p=8155). 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 resulting from the extrapolation. I would always ensure that (1) the initial pitch angle set within the ElastoDyn primary input file matches the pitch specified within the Simulink model at time zero and (2) the pitch angle changes are defined smoothly within Simulink.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Barry.Dolan
Posts: 6
Joined: Mon Oct 24, 2011 5:23 am
Organization: Solutions 4 Energy
Location: Germany

Re: Pitch control error

Postby Barry.Dolan » Tue Dec 11, 2018 2:23 am

Dear Jason,

thanks for your answer. Setting the initial blade pitch angles in the Elastodyn input file solved the problem.

After reading the other topics which you linked, I also found that by setting the interpolation type to linear the oscillations I observed with the quadratic interpolation are much smaller. So small in fact that they do not cause any error during the simulation. This may be helpful for someone who wants to calculate a Cp surface using inputs from Simulink, since it means that it won't be necessary to create a separate Elastodyn input file (in which the inital pitch angle is set) for every single pitch angle simulated.

Best regards,
Barry.

Jason.Jonkman
Posts: 4843
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: Pitch control error

Postby Jason.Jonkman » Tue Dec 11, 2018 6:31 am

OK, thanks for the advice!
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov


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