problem with system identification

Dear All

i am trying to make system identification for the following system

The problem i can not get good model fit for the system(Note : there is a limit cycle in the system with some input range). what is your suggetions? I appreciate your time and help

Regards

[Nijas palakkal]

please explain what exactly your system is.?

linear ,non-linear ?

[Ahmed Hamed]

Its second order nonlinear system (torsional spring system). The data was obtained from experiment.