3. Model the control system

3.1 Automatic controllers

First, we need to identify the system's various operating modes in order to build the PLC. For our example of a tank heating system, the different modes are "off" and "on". The "on" mode can be broken down into sub-modes such as fast and slow heating.

A graphical representation of these different modes and the conditions for their passage is often made using Mathworks' Stateflow tool. Stateflow is an extension of the Simulink dynamic systems modeling software, with a design environment for the development of state machines.