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Distributed control systems (DCS) use decentralized elements or subsystems to control distributed processes or manufacturing systems. They offer flexibility, extended equipment life, simplicity of new equipment integration, and centralized maintenance when used in an industrial environment.

 

Characteristics and Design

 

A distributed control system involves the placement of multiple controllers within a plant or manufacturing process. The controllers are networked to a central console. DCSs aim to centralize plant operations to allow control, monitoring, and reporting of individual components and processes at a single location.

 

Components

 

DCSs are by definition hierarchical systems, although not all systems share an identical hierarchy.

 

The image below shows a typical DCS. Individual controllers, supervised by master controllers, make up the lowest "field" or "plant" level of the hierarchy. The master controllers connect to individual computers and servers, which are further connected to video output devices and a human machine interface (HMI), which is the actual point of user control. DCSs are usually networked using standard protocols such as PROFIBUS and Ethernet, the latter of which is used in this particular system. 

 

distributed control system selection guide

Image credit: Alstom

 

It is important to note that many DCS components can also operate as standalone devices. While a DCS ultimately governs the functionality of its networked components, the same components can often be reprogrammed for use in other applications.

 

Redundancy

 

Most distributed control systems are designed with redundant elements. Redundant engineering increases a system's reliability by using backup processors in case of primary processor failure. Redundant elements are necessary in DCSs because for two main reasons:

 

  • Many DCSs control safety-critical processes in which failure or outage of equipment could cause personal injury or loss of life. A petroleum refinery is a good example of safety-critical plant. In such an environment, a control system governs flares that constantly burn gas. If the control system fails and the flares cease burning, gas collects and pools, causing an extremely dangerous situation.
  • Redundancy increases equipment reliability, leaving the DCS operator to concentrate on displays, software, and applications. Because DCS systems require near-constant operator interaction at the HMI, redundancy is crucial.

Applications

 

Distributed control systems are most frequently used in batch-oriented or continuous process operations, such as oil refining, power generation, petrochemical manufacturing, papermaking, food and beverage manufacturing, pharmaceutical production, and cement processing. DCSs may control any of a number of different equipment types, including:

 

  • Variable speed drives
  • Quality control systems
  • Motor control centers (MCC)
  • Kilns
  • Manufacturing equipment
  • Mining equipment

Relationship with PLCs and SCADA

 

DCS vs. PLC

 

DCSs and programmable logic controllers (PLC) were historically used in dissimilar applications, but this distinction has blurred more recently. In fact, PLC and DCS architectures are often difficult to distinguish and use many of the same components, including field sensors, I/O modules, HMIs, and supervisory control systems. Distributed control systems are typically much more expensive and control continuous processes and critical applications, while PLCs are used for high-speed machine control.

 

The table below describes some differences between and advantages of using a DCS and PLC.

 

Type

Product

Value

Control center

Operator

System

Customization

Engineering

PLC

Manufacturing of specific items; simple batch control

Low component value; easy restart after outage; downtime does not damage process equipment

Controller is heart of system

Operator intervenes primarily to handle errors

Fast logic (~10 ms); no redundancy required; alarm sounds upon error detection

Completely customizable; customization typically required

Bottom-up design; flexible; generic solution

DCS

Transformation of raw materials; complex batch control

High batch value; downtime can damage equipment; difficult restart after outage

HMI is heart of system

Operator interaction is continuous; failure of HMI can result in outage

Slower loop control (~100-500 ms); redundancy often required; alarm sounds prior to error

Customized using functional blocks; algorithms may not vary with different applications

Top-down design; functional “out of the box”; pre-defined functions specific to application

 

vs. SCADA

 

The relationship between DCSs and supervisory control and data acquisition (SCADA) is similar to that of DCSs and PLCs. In the past the two systems were used for disparate applications, but more recently their implementations and characteristics have appeared increasingly similar.

 

Important differences between DCS and SCADA are:

 

  • DCS is process-oriented; SCADA is data-gathering-oriented
  • SCADA maintains speed by storing good values and data for use during outages; DCS relies on constant stream of data for control
  • DCS uses closed-loop control; SCADA uses human supervisory control

 

The image below shows the implementation of a DCS and SCADA system to integrate two automation systems. This setup is common in applications which require simultaneous process and power automation.

 

distributed control systems dcs selection guide

Image credit: ABB / Process Automation Insights

 

Standards

 

Distributed control systems may be designed and used with the aid of published standards and specifications. Some example DCS standards are:

 

IEC—Distributed control and filtering for industrial systems (handbook)

SAE AS5370—Fiber optic data bus for distributed aircraft control systems

 

References

 

Dresser-Rand—The case for DCS in turbomachinery (pdf)

Siemens—DCS or PLC? (pdf)

Synergist—DCS vs. SCADA systems

 

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