The term Control System, in its broadest sense, can apply to a very wide range of different systems that operate at different levels within an industrial system. One good way to classify different types of control systems is in terms of functionality:

• Level 0 - Emergency Shut Down System (ESD). Contains all the fail-safe mechanisms for the system
• Level 1 - Low Level Control. Direct control of actuators
• Level 2 - Supervisory Control. Co-ordination of several, or plant-wide, low level controllers
• Level 3 - Management Systems. Including resource planning systems and management information systems

These levels have a very different emphasis for each type of control system application. For example simple servo control applications have very little in terms of the upper-most level, whereas a within a Chemical plant the level 0 systems are critical to plant safety, but level 3 systems are the ones that improve the economics of the plant.

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