In the figure there is represented a step response of a mechanical system. I created a step variation in the excitation of a DC generator. This lead to a step variation in the torque developed by that generator. I measured this torque with a arm connected to the stator and to a load cell. Is this thing normal?
I tried to identify the transfer function of this system have as output the plot from the figure and a step change as input. I used IDENT from MATLAB and got a transfer function with 3 zeros and 3 poles with a fit of 85%. I used this transfer function with a sin wave input and computed the phase shift between the input and the output. This shift is very small, 2-3°. Is this thing normal? I see that my system has a big delay from the step response but with sin wave response the delay is very small. Is it possible that the procedure for obtaining the transfer function is not correct?