I am trying to do PID control for my servo electronic motor system. This system uses SSI encoder output for feedback mechanism. I will calculate PID parameters according to ziegler nichols tuning methods. However I must know transfer function of this system while determining PID parameters. So how can I extract transfer function of my system? Is there any way by using MATLAB/SIMULINK or Labview? I think I need a methodology for this? I do not know how can I do. Can anyone explain clearly.
As I remember (learned it 15 yrs ago) You need to have an experimental data, which will help you to find "linear" sections and the their limits (with taking into account the things like hysteresis), so you will be able to find transfer function for each band.
P.S. Why do you think that the servo motor system is nonlinear? Please describe the the mechanism to which motor is connected? What parameter you want to control?