Can anybody advise me on how to establish a transfer function for a butterfly valve. I am trying to model a system using matlab/simulink but I do not know how to integrate the valve. I am controlling the pressure of airflow in a duct with a butterfly valve and an analogue electronic controller. The valve will be actuated via an electric motor and gearing. Is there a way to model the valve with respect to the properties/dynamics of the air flowing through it?