Is it possible to update the force in real-time in STRUCTURAL CONTROL (StC_DOF_MODE=4)?

[Billy0710]

Dear openfast team,

Thank you for all your efforts in making openfast such a great application. I tried many ways to input my control to openfast, but passing "5: Force determined by external DLL" was difficult for me, so I was wondering if openfast can read time-force series data in parallel?

[jjonkman]

Dear @Billy0710,

What difficulty did you have with StC_DOF_MODE = 5?

I'm not sure what you mean when ask if "OpenFAST can read time-force series data in parallel", but the Structural Control (StC) submodel of ServoDyn can certainly be used to specify multiple point force time series across different members (blades, nacelle, tower, substructure) and different points in each member, at the same time, when StC_DOF_MODE = 4.

Best regards,

[RBergua]

I'm not sure if this is what @Billy0710 means by "read time-force series data", but it would be really good if the structural control in ServoDyn could take an available channel from OpenFAST (e.g., tower top fore-aft deflection, rotor speed...) to compute a parametric load that gets then applied somewhere else in the model. Nowadays, the user can only provide a time series load that has been precomputed (i.e., not online computation).

[Billy0710]

Hi guys,

Sorry for the confusion. I wonder that If I use the StC_DOF_MODE = 4, which means I only can provide the time series load in advance? But my controller need the feedback from the openfast system every time step. Or can I import my control input somehow (like a floating platform)? Because my initial idea was that if I could update the PRESCRIBED TIME SERIES file in real time, my control input would be input into the stucture at each time step, but through your solution it seems that this is not possible.

And I have already got the control input from the ROSCO controller(Simulink), which can generate a load. And there is a problem that I can't find the proper junction for that to import it to Openfast...

With regards,

[andrew-platt]

Hi @Billy0710,

The StC_DOF_MODE = 4 option is only usable with an externally provided pre-computed time series of applied loads. This option does not work with the controller.

If you need to apply a dynamically changing load, you can supply this from the turbine DLL controller. However, ROSCO does not natively support this option, so you would need to modify ROSCO with the logic you need. To use this, you will need to set StC_DOF_MODE = 5, StC_CMODE = 5, and assign an StC_CChan group number. Then you can pass in the appropriate values through the avrSWAP array. The avrSWAP entries available are:

• J+1 - StC_Disp_X
• J+2 - StC_Disp_Y
• J+3 - StC_Disp_Z
• J+4 - StC_Vel_X
• J+5 - StC_Vel_Y
• J+6 - StC_Vel_Z
• J+7 - StC_Stiff_X (TMD override spring constant)
• J+8 - StC_Stiff_Y (TMD override spring constant)
• J+9 - StC_Stiff_Z (TMD override spring constant)
• J+10 - StC_Damp_X (TMDoverride damping constant)
• J+11 - StC_Damp_Y (TMD override damping constant)
• J+12- StC_Damp_Z (TMD override damping constant)
• J+13 - StC_Brake_X (TMD braking force)
• J+14 - StC_Brake_Y (TMD braking force)
• J+15 - StC_Brake_Z (TMD braking force)
• J+16 - StC_Force_X (external additional force)
• J+17 - StC_Force_Y (external additional force)
• J+18 - StC_Force_Z (external additional force)
• J+19 - Reserved for future
• J+20 - Reserved for future
  where the J = 2800 + (StC_CChan-1)*20. The first 6 entries (J+1 to J+6) are passed from the Structural control module to the controller when StC_DOF_MODE = 1,2, and channels J+7 to J+15 can be used to control a TMD response directly.

For your case when you want to apply an external load that varies with time, you could set StC_CChan = 1 and pass in the external forces through channels:

• 2816 - StC_Force_X for StC control group 1
• 2817 - StC_Force_Y for StC control group 1
• 2818 - StC_Force_Z for StC control group 1
  and for a second independently controlled StC using StC_CChan = 2, pass the external forces through channels
• 2836 - StC_Force_X for StC control group 2
• 2837 - StC_Force_Y for StC control group 2
• 2838 - StC_Force_Z for StC control group 2

As mentioned above, you will need to modify the controller to output these channels to the StC. Two words of caution though:

1. we have never fully tested the above approach so there may still be some bugs in the logic with it
2. applying an external force like this is not energy conserving in the turbine system. This is the equivalent of attaching a rope from the nacelle to a ground based actuator that applies the force.

Regards,
Andy

PS: details on the code implementation (for developers reference later):

• avrSWAP entry locations: BladedInterface_EX.f90 65-67 (StCCtrl_StartIdx value for start index)
• avrSWAP entry description: BladedInterface_EX.f90 350-370
• Set motion info to DLL avrSWAP: BladedInterface_EX.f90 591-613
• Get channels from DLL avrSWAP: 674-689 (typo in description of channel location) (search StCCmdForce)
• Setting force: ServoDyn.f90: 2086 (u_StC%CmdForce, StC_CmdForce variable - logic for ramping between timesteps, so StCCmdForce and PrevStCCmdForce variables also exist)
• Setting force in structural control: StrucCtrl.f90 1670 (F_Ctrl = u%CmdForce)
• Application of force in structural control code: StrucCtrl.f90 (search for m%F_ext in equations)
  TODO: looking at the naming conventions used, there is some room for cleanup to make it easier to follow through the code.