Distributed control system: Difference between revisions

Revision as of 12:13, 29 November 2022

Revision as of 12:32, 29 November 2022

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{{Use American english|date=February 2018}}

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

{{Manipulating}}

A '''distributed controledcontrol system ''' ('''dcsDCS''') is a computerized [[controllcontrol system]] for a proccesorprocessor plant usually with nanymany [[control loop]]s, in which automatic controllers are distributed throughout the system, but there is no 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 localisinglocalizing 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|Supervisory control and data acquisition (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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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 computerisedcomputerized 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 industrialisedindustrialized 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

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==History==

[[File:Kontrollrom Tyssedal.jpg|thumb|A pre-DCS era central control room. Whilst the controls are centralisedcentralized in one place, they are still discrete and not integrated into one system.]]

[[File:Leitstand 2.jpg|thumb|A DCS control room where plant information and controls are displayed on computer graphics screens. The operators are seated as they can view and control any part of the process from their screens, whilst retaining a plant overview.]]

===Evolution of process control operations===

Process control of large industrial plants has evolved through many stages. Initially, control would be from panels local to the process plant. However this required a large manpower resource to attend to these dispersed panels, and there was no overall view of the process. The next logical development was the transmission of all plant measurements to a permanently-manned central control room. Effectively this was the centralisationcentralization of all the localisedlocalized panels, with the advantages of lower manning levels and easier overview of the process. Often the controllers were behind the control room panels, and all automatic and manual control outputs were transmitted back to plant. However, whilst providing a central control focus, this arrangement was inflexible as each control loop had its own controller hardware, and continual operator movement within the control room was required to view different parts of the process.

With the coming of electronic processors and graphic displays it became possible to replace these discrete controllers with computer-based algorithms, hosted on a network of input/output racks with their own control processors. These could be distributed around plant, and communicate with the graphic display in the control room or rooms. The distributed control system was born.