Content deleted Content added
Plastikspork (talk | contribs) Rv |
m Clean up spacing errors around ref tags., replaced: /ref>a → /ref> a |
||
Line 1:
{{Use British English|date=February 2018}}▼
{{Short description|Computerized control systems with distributed decision-making}}
▲{{Use British English|date=February 2018}}
{{Manufacturing}}
A '''distributed control system''' ('''DCS''') is a computerised [[control system]] for a process or plant usually with many [[control loop]]s, in which autonomous 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 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|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.
Line 8:
==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;
Line 18:
* 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.
Line 28:
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
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).
Line 41:
==Typical applications==
Distributed control systems (DCS) are dedicated systems used in manufacturing processes that are continuous or batch-oriented.
Processes where a DCS might be used include:
Line 68:
===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 centralisation of all the localised 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.
The introduction of DCSs allowed easy interconnection and re-configuration of plant controls such as cascaded loops and interlocks, and easy interfacing with other production computer systems. It enabled sophisticated alarm handling, introduced automatic event logging, removed the need for physical records such as chart recorders, allowed the control racks to be networked and thereby located locally to plant to reduce cabling runs, and provided high level overviews of plant status and production levels.
Line 88:
}}</ref>
In 1975, both [[Yamatake-Honeywell]]<ref>{{Cite web|url=https://www.azbil.com/corporate/company/history.html|title = Group History | Azbil Corporation Info | About the azbil Group | Azbil Corporation (Former Yamatake Corporation)}}</ref> and Japanese electrical engineering firm [[Yokogawa]] introduced their own independently produced DCS's - TDC 2000 and CENTUM systems, respectively. US-based Bristol also introduced their UCS 3000 universal controller in 1975. In 1978 [[Valmet]] introduced their own DCS system called Damatic (latest generation named Valmet DNA<ref name="Valmet">[https://www.valmet.com/automation/control-systems/] Valmet DNA</ref>). In 1980, Bailey (now part of ABB<ref>[http://www.abb.com/controlsystems] INFI 90</ref>) introduced the NETWORK 90 system, Fisher Controls (now part of [[Emerson Electric]]) introduced the PROVoX system, [[Fischer & Porter Company]] (now also part of ABB<ref>[http://www.abb.com/product/us/9AAC115762.aspx] DCI-4000</ref>) introduced DCI-4000 (DCI stands for Distributed Control Instrumentation).
The DCS largely came about due to the increased availability of microcomputers and the proliferation of microprocessors in the world of process control. Computers had already been applied to process automation for some time in the form of both [[direct digital control]] (DDC) and setpoint control. In the early 1970s [[Taylor Instrument Company]], (now part of ABB) developed the 1010 system, Foxboro the FOX1 system, Fisher Controls the DC<sup>2</sup> system and [[Bailey Controls]] the 1055 systems. All of these were DDC applications implemented within minicomputers ([[Digital Equipment Corporation|DEC]] [[PDP-11]], [[Varian Data Machines]], [[MODCOMP]] etc.) and connected to proprietary Input/Output hardware. Sophisticated (for the time) continuous as well as batch control was implemented in this way. A more conservative approach was setpoint control, where process computers supervised clusters of analog process controllers. A workstation provided visibility into the process using text and crude character graphics. Availability of a fully functional graphical user interface was a way away.
Line 122:
The latest developments in DCS include the following new technologies:
# [[Wireless]] systems and protocols <ref>
# [[Remote data transmission|Remote transmission]], logging and data historian
# Mobile interfaces and controls
| |||