Editing Industrial process control (section)

==Control loops==
[[File:Smart current loop positioner.png|thumb|Example of a continuous flow control loop. Signalling is by industry standard 4-20 mA current loops, and a "smart" [[control valve|valve positioner]] ensures the [[control valve]] operates correctly.]]
The fundamental building block of any industrial control system is the [[control loop]], which controls just one process variable. An example is shown in the accompanying diagram, where the flow rate in a pipe is controlled by a [[PID controller]], assisted by what is effectively a cascaded loop in the form of a valve servo-controller to ensure correct valve positioning.

Some large systems may have several hundreds or thousands of control loops. In complex processes the loops are interactive, so that the operation of one loop may affect the operation of another. The system diagram for representing control loops is a [[Piping and instrumentation diagram]].

Commonly used control systems include [[programmable logic controller]] (PLC), [[Distributed Control System]] (DCS) or [[SCADA]].

[[File:Auxostat schematic.svg|thumb|Example of level control system of a [[continuous stirred-tank reactor]]. The flow control into the tank would be cascaded off the level control.]]

A further example is shown. If a control valve were used to hold level in a tank, the level controller would compare the equivalent reading of a level sensor to the level setpoint and determine whether more or less valve opening was necessary to keep the level constant. A cascaded flow controller could then calculate the change in the valve position.