Hi Tarik,
One option if you do not have the model order readily available, is to do plant I/O experiments to determine the nature of response of the system, say, for a step input. From that you could see if you system behaves like a first or a second order system.
Simulink Control Design has a couple of disturbance rejection capabilities that you use to kickstart your work. Please see them below:
- Active disturbance rejection block
- Disturbance compensator block (you could use this block with any type of controller to estimate and reject disturbances)
HTH,
Sid
