I am trying to find the transfer function of a plant that contains numerous equipment including a heater, several tanks, pumps, membranes etc.
I need to design 2 PID controllers: the first one to control the temperature of the stream entering one of the membranes, and the second PID controller needs to maintain the solution concentration in one of the tanks.
I have seen that there is a system identification tool in Matlab that I can use to find the plant transfer function. The problem is, I am not clear on exactly what the plant transfer function should describe. Should it represent the whole plant (e.g. all the tanks, pumps etc.) or only the part of the plant that I am interested in to be able to design the controllers (e.g. resulting in two different plant transfer function equations for each controller).
Thanks in advance.