In a process plant, we typically have a number of control goals that require different algorithms and types of information from the process; from the level closest to the process to the plant-wide control. All these activities ought to be connected; hence some kind of hierarchy is necessary.
Local Control
Local control uses information directly collected from the process, as fast as possible, to generate the reaction of the control to process changes in order to fulfill the tracking or regulatory control goals. It also includes logic control to handle automata.
Supervisory Control
Supervisory control level evaluates the behaviour of the local controls and commands change in parameters, structures or components. It may include adaption as well as emergency actions.
Related: Basic Features of Distributed Control Systems (DCS)
Coordinating Control
Coordinating control evaluates the performances of the process and determines the set-points (objectives) for the lower level controls. It may include optimization routines to compute the best operating conditions.
Plant-wide Control
This control level considers the constraints among the interconnected processes to optimise the whole plant behaviour. It may include management and policy criteria to define goals for the lower controls.
Also read: Basic Steps to Consider in Designing a Control System
