Editing Distributed control system (section)

==Structure==
[[File:Functional levels of a Distributed Control System.svg|thumb|500px|Functional levels of a manufacturing control operation]]
The key attribute of a DCS is its reliability due to the distribution of the control processing around nodes in the system. This mitigates a single processor failure. If a processor fails, it will only affect one section of the plant process, as opposed to a failure of a central computer which would affect the whole process. This distribution of computing power local to the field Input/Output (I/O) connection racks also ensures fast controller processing times by removing possible network and central processing delays.

The accompanying diagram is a general model which shows functional manufacturing levels using computerised control.

Referring to the diagram;

* Level 0 contains the field devices such as flow and temperature sensors, and final control elements, such as [[control valve]]s
* Level 1 contains the industrialised Input/Output (I/O) modules, and their associated distributed electronic processors.   
* Level 2 contains the supervisory computers, which  collect information from processor nodes on the system, and provide the operator control screens. 
* Level 3 is the production control level, which does not directly control the process, but is concerned with monitoring production and monitoring targets
* Level 4 is the production scheduling level.

Levels 1 and 2 are the functional levels of a traditional DCS, in which all equipment are part of an integrated system from a single manufacturer.

Levels 3 and 4 are not strictly [[process control]] in the traditional sense, but where production control and scheduling takes place.

===Technical points===
[[File:Smart current loop positioner.png|thumb|Example of a continuous flow control loop. Signalling is by industry standard 4–20&nbsp;mA current loops, and a "smart" [[control valve|valve positioner]] ensures the [[control valve]] operates correctly.]]
The processor nodes and operator [[graphical user interface|graphical displays]] are connected over proprietary or industry standard networks, and network reliability is increased by dual redundancy cabling over diverse routes. This distributed topology also reduces the amount of field cabling by siting the I/O modules and their associated processors close to the process plant.

The processors receive information from input modules, process the information and decide control actions to be signalled by the output modules. The field inputs and outputs can be [[analog signal]]s e.g. [[current loop|4–20&nbsp;mA DC current loop]] or two-state signals that switch either "on" or "off", such as relay contacts or a semiconductor switch.

DCSs are connected to sensors and actuators and use [[Setpoint (control system)|setpoint control]] to control the flow of material through the plant. A typical application is a [[PID controller]] fed by a flow meter and using a [[control valve]] as the final control element. The DCS sends the setpoint required by the process to the controller which instructs a valve to operate so that the process reaches and stays at the  desired setpoint. (see 4–20&nbsp;mA schematic for example).

Large oil refineries and chemical plants have several thousand I/O points and employ very large DCS. Processes are not limited to fluidic flow through pipes, however, and can also include things like [[paper machine]]s and their associated quality controls, [[Adjustable-speed drive|variable speed drives]] and [[Motor controller|motor control centers]], [[cement kiln]]s, [[Mining|mining operations]], [[Extractive metallurgy|ore processing]] facilities, and [[Et cetera|many others]].

DCSs in very high reliability applications can have dual redundant processors with "hot" switch over on fault, to enhance the reliability of the control system.

Although 4–20&nbsp;mA  has been the main field signalling standard, modern DCS systems can also support [[fieldbus]] digital protocols, such as Foundation Fieldbus, profibus, HART, [[modbus]], PC Link, etc.

Modern DCSs also support [[Artificial neural network|neural networks]] and [[fuzzy logic]] applications. Recent research focuses on the synthesis of optimal distributed controllers, which optimizes a certain [[H-infinity methods in control theory|H-infinity]] or the H 2 control criterion.<ref>{{Cite journal|title = Distributed Control Design for Spatially Interconnected Systems|last = D'Andrea|first = Raffaello |date = 9 September 2003|journal = IEEE Transactions on Automatic Control|volume = 48|issue = 9|pages = 1478–1495|doi = 10.1109/tac.2003.816954 |citeseerx = 10.1.1.100.6721}}</ref><ref>{{Cite journal|url = http://resolver.tudelft.nl/uuid:2a1e3740-454f-4a1e-bd0d-cda8846eadae|title = Distributed Control for Identical Dynamically Coupled Systems: A Decomposition Approach|last = Massiaoni|first = Paolo|date = 1 January 2009|journal = IEEE Transactions on Automatic Control|volume = 54|pages = 124–135|doi = 10.1109/tac.2008.2009574 |s2cid = 14384506}}</ref>