Distributed control system: Difference between revisions

Revision as of 13:52, 18 September 2019

Revision as of 12:01, 27 September 2019

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{{Short description|Computerized control systems with distributed decision-making}}

{{Manufacturing}}

A '''distributed control system''' ('''DCS''') is a computerised [[control system]] for a process or plant usually with a large number ofmany [[control loop]]s, in which autonomous controllers are distributed throughout the system, but there is central operator supervisory control. This is in contrast to systems that use centralized controllers; either discrete controllers located at a central control room or within a central computer. The DCS concept increases reliability and reduces installation costs by localising control functions near the process plant, with remote monitoring and supervision.

Distributed control systems first emerged in large, high value, safety critical process industries, and were attractive because the DCS manufacturer would supply both the local control level and central supervisory equipment as an integrated package, thus reducing design integration risk. Today the functionality of [[SCADA]] and DCS systems are very similar, but DCS tends to be used on large continuous process plants where high reliability and security is important, and the control room is not geographically remote.

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* 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.

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===Technical points===

[[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 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|analog signals]] e.g. [[current loop|4–20 mA DC current loop]] or 2 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 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 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., and other digital communication protocols such as [[modbus]].

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|url = |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 }}</ref>