After a long time doing the simulation using FAST-SC, I encountered a problem about the setting of variables “TmdXFext”. I know it is not very appropriate to post a problem about FAST-SC here but I have tried to reach Prof Lackner and he said that he have not using FAST-SC for years hence can not remember some details. I post it here just hoping someone using FAST-SC may have done similar simulation so that we could discuss it.
Let my explain what I have done first, I am doing the simulink of FAST-SC mainly using the port “TmdXFext” as the input. Meanwhile, I have set the spring and damper coefficients of TmdX to zero in the primary input files and set the control mode of TmdX to get inputs from simulink. I did not enable the DOF of TmdY as I only want to do some test of TmdX. I set TmdXDxn and TmdXVxn as outputs and multiplied them separately with 64000N/m and 10000Ns/m and connect them to the port “TmdXFext” as feedback just to mimic the existence of TmdX. It matches pretty well (only very little difference) when I connect both of them to the port “TmdXFext” . But when I only connect “TmdXDxn * 64000” to the port, I find that the motion of tower top and TmdX is different with the situation that I set the coefficients of spring to 64000N/m and leave the coefficients of damper to zero as well as no force in"TmdXFext" . From my opinion, the only difference is I either set the coefficients of spring to 64000N/m in simulink or I extract the TmdXDxn which is the displacement of TmdX and multiply it by 64000 and feed to TmdXFext. All other settings are the same so I do not know why there is a big difference as shown in the following picture.
The blue line is the scenario I set the coefficients of spring to 64000N/m and the red line is the scenario I extract TmdXDxn and multiply it by 64000 and feed it to the port “TmdXFext”.
By the way, I tried the situation for damper and I found them match well (only very little difference).
I do not know why there is a huge difference and I am wondering if I could do that to mimic the existence of spring? If not, what is the reason? Thank you very much