Phase shift in blade root flap wise bending moment

1

Dear sir,
I am trying to implement individual pitch controller on a linearized model. I have a doubt that, If we apply base line(PI) controller to fast linearized model , and regulate the generator speed using collective blade pitch control , can we expect to get 120 degree phase shift in between the three blade root out of plane bending moments ? thank you in advance.

                 thank and regards
2

Dear @Suresh.Nakkala,

I’m not sure I understand your question. Is your linearized model periodic or azimuth averaged? Are you referring to a 120-degree phase shift due to gravity loads, shear, skew, etc and are these effects captured by your linear model?

Best regards,

3

Dear sir
I have linearized the model from the bladed 4.10 .
I have given the azimuth step 10 degree .so it generated linearized model for each (360/10 =36) .
I am applying collective pitch controller for one out of the 36 models using simple pi controller (with help of generator speed regulation).I have doubt that which one i can consider out of 36 linearized models. but any way i chose one and i applied collective pitch controller but i am not getting the 120 degree phase shift in between the blade bending moments when the wind shear effect is present. Yes sir, I have captured linearized models with wind shear present. thank you in advance.
than and regards

4

Dear @Suresh.Nakkala,

To design a pitch controller for a 3-bladed rotor, I would normally expect that you’d:

  1. Extract a periodic linearized model from OpenFAST (e.g., as you said, in 36 increments of 10 degrees)
  2. Apply MBC3 to each of these linear models
  3. Azimuth average the MBC3-transformed linear models, resulting in a linear time-invariant (LTI) model
  4. Eliminate the generator azimuth state from the LTI model
  5. Design your controller

When you simulate with the LTI model, I would expect that you’d post-process results in the rotating frame of reference (e.g., blade root flapwise bending moment) as follows:

  1. Compute the MBC-transformed outputs from the LTI model
  2. Integrate the generator speed to get the azimuth angle (which was eliminated as a state from the LTI model)
  3. Apply the inverse MBC3 to transform the outputs back into the rotating frame of reference

Are these the steps you have followed?

This process has been discussed previously on this forum, e.g., see: questions about reference control input and FAST linearization V7 - #7 by Yanhua.Liu.

Best regards,