A distributed control system refers to a control system of a process plant and industrial process
wherein control elements are not only located in central location but are also distributed
throughout the system with each component sub-system controlled by one or more controllers
so the intelligence is distributed across the sections of the plant. DCS follows hierarchy
in its control philosophy with various function spread across .
DCS is a computerized control system used to automate processes in various industries.
The entire system of controllers is connected by networks for communication and monitoring.
DCS is a very broad term used to monitor and control distributed equipments in process
plants and industrial processes . Chemical plants
Petrochemical industries and refineries Boiler controls and power plant systems
Nuclear power plants Environmental control systems
Water management systems Oil refining plants
Metallurgical process plants Chemical plants
Pharmaceutical manufacturing Sugar plants
Dry cargo and bulk oil carrier ships
Elements
A DCS typically uses custom designed processors as controllers and uses both proprietary interconnections
and standard communications protocol for communication. Input and output modules form component parts
of the DCS. The processor receives information from input modules and sends information to
output modules. The input modules receive information from input instruments in the
process and the output modules transmit instructions to the output instruments in the field. The
inputs and outputs can be either analog signal which are continuously changing or discrete
signals which are 2 state either on or off . Computer buses or electrical buses connect
the processor and modules through multiplexer or demultiplexers. Buses also connect the
distributed controllers with the central controller and finally to the Human–machine interface
or control consoles. See Process automation system.
The elements of a DCS may connect directly to physical equipment such as switches, pumps
and valves and to Human Machine Interface via SCADA. The differences between a DCS and
SCADA is often subtle, especially with advances in technology allowing the functionality of
each to overlap. Applications
Distributed control systems are dedicated systems used to control manufacturing processes
that are continuous or batch-oriented, such as oil refining, petrochemicals, central station
power generation, fertilizers, pharmaceuticals, food and beverage manufacturing, cement production,
steelmaking, and papermaking. DCSs are connected to sensors and actuators and use setpoint
control to control the flow of material through the plant. The most common example is a setpoint
control loop consisting of a pressure sensor, controller, and control valve. Pressure or
flow measurements are transmitted to the controller, usually through the aid of a signal conditioning
input/output device. When the measured variable reaches a certain point, the controller instructs
a valve or actuation device to open or close until the fluidic flow process reaches the
desired setpoint. Large oil refineries have many thousands of I/O points and employ very
large DCSs. Processes are not limited to fluidic flow through pipes, however, and can also
include things like paper machines and their associated quality controls, variable speed
drives and motor control centers, cement kilns, mining operations, ore processing facilities,
and many others. A typical DCS consists of functionally and/or
geographically distributed digital controllers capable of executing from 1 to 256 or more
regulatory control loops in one control box. The input/output devices can be integral with
the controller or located remotely via a field network. Today’s controllers have extensive
computational capabilities and, in addition to proportional, integral, and derivative
control, can generally perform logic and sequential control. Modern DCSs also support neural networks
and fuzzy application. DCSs are usually designed with redundant processors
to enhance the reliability of the control system. Most systems come with canned displays
and configuration software which enables the end user to set up the control system without
a lot of low level programming. This allows the user to better focus on the application
rather than the equipment, although a lot of system knowledge and skill is still required
to support the hardware and software as well as the applications. Many plants have dedicated
groups that focus on this task. These groups are in many cases augmented by vendor support
personnel and/or maintenance support contracts. DCSs may employ one or more workstations and
can be configured at the workstation or by an off-line personal computer. Local communication
is handled by a control network with transmission over twisted pair, coaxial, or fiber optic
cable. A server and/or applications processor may be included in the system for extra computational,
data collection, and reporting capability. History
Early minicomputers were used in the control of industrial processes since the beginning
of the 1960s. The IBM 1800, for example, was an early computer that had input/output hardware
to gather process signals in a plant for conversion from field contact levels and analog signals
to the digital domain. The first industrial control computer system
was built 1959 at the Texaco Port Arthur, Texas, refinery with an RW-300 of the Ramo-Wooldridge
Company The DCS was introduced in 1975. Both Honeywell
and Japanese electrical engineering firm Yokogawa introduced their own independently produced
DCSs at roughly the same time, with the TDC 2000 and CENTUM systems, respectively. US-based
Bristol also introduced their UCS 3000 universal controller in 1975. In 1978 Metso(known as
Valmet in 1978) introduced their own DCS system called Damatic. In 1980, Bailey introduced
the NETWORK 90 system, Fisher Controls introduced the PROVoX system, Fischer & Porter Company
introduced DCI-4000. 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 and set point control. In the early 1970s Taylor Instrument
Company, developed the 1010 system, Foxboro the FOX1 system, Fisher Controls the DC2 system
and Bailey Controls the 1055 systems. All of these were DDC applications implemented
within minicomputers and connected to proprietary Input/Output hardware. Sophisticated continuous
as well as batch control was implemented in this way. A more conservative approach was
set point control, where process computers supervised clusters of analog process controllers.
A CRT-based workstation provided visibility into the process using text and crude character
graphics. Availability of a fully functional graphical user interface was a way away.
Central to the DCS model was the inclusion of control function blocks. Function blocks
evolved from early, more primitive DDC concepts of "Table Driven" software. One of the first
embodiments of object-oriented software, function blocks were self-contained "blocks" of code
that emulated analog hardware control components and performed tasks that were essential to
process control, such as execution of PID algorithms. Function blocks continue to endure
as the predominant method of control for DCS suppliers, and are supported by key technologies
such as Foundation Fieldbus today. Midac Systems, of Sydney, Australia, developed
an objected-oriented distributed direct digital control system in 1982. The central system
ran 11 microprocessors sharing tasks and common memory and connected to a serial communication
network of distributed controllers each running two Z80s. The system was installed at the
University of Melbourne. Digital communication between distributed
controllers, workstations and other computing elements was one of the primary advantages
of the DCS. Attention was duly focused on the networks, which provided the all-important
lines of communication that, for process applications, had to incorporate specific functions such
as determinism and redundancy. As a result, many suppliers embraced the IEEE 802.4 networking
standard. This decision set the stage for the wave of migrations necessary when information
technology moved into process automation and IEEE 802.3 rather than IEEE 802.4 prevailed
as the control LAN. The Network Centric Era of the 1980s
In the 1980s, users began to look at DCSs as more than just basic process control. A
very early example of a Direct Digital Control DCS was completed by the Australian business
Midac in 1981–82 using R-Tec Australian designed hardware. The system installed at
the University of Melbourne used a serial communications network, connecting campus
buildings back to a control room "front end". Each remote unit ran 2 Z80 microprocessors
whilst the front end ran 11 in a Parallel Processing configuration with paged common
memory to share tasks and could run up to 20,000 concurrent controls objects.
It was believed that if openness could be achieved and greater amounts of data could
be shared throughout the enterprise that even greater things could be achieved. The first
attempts to increase the openness of DCSs resulted in the adoption of the predominant
operating system of the day: UNIX. UNIX and its companion networking technology TCP-IP
were developed by the US Department of Defense for openness, which was precisely the issue
the process industries were looking to resolve. As a result suppliers also began to adopt
Ethernet-based networks with their own proprietary protocol layers. The full TCP/IP standard
was not implemented, but the use of Ethernet made it possible to implement the first instances
of object management and global data access technology. The 1980s also witnessed the first
PLCs integrated into the DCS infrastructure. Plant-wide historians also emerged to capitalize
on the extended reach of automation systems. The first DCS supplier to adopt UNIX and Ethernet
networking technologies was Foxboro, who introduced the I/A Series system in 1987.
The application-centric era of the 1990s The drive toward openness in the 1980s gained
momentum through the 1990s with the increased adoption of commercial off-the-shelf components
and IT standards. Probably the biggest transition undertaken during this time was the move from
the UNIX operating system to the Windows environment. While the realm of the real time operating
system for control applications remains dominated by real time commercial variants of UNIX or
proprietary operating systems, everything above real-time control has made the transition
to Windows. The introduction of Microsoft at the desktop
and server layers resulted in the development of technologies such as OLE for process control,
which is now a de facto industry connectivity standard. Internet technology also began to
make its mark in automation and the DCS world, with most DCS HMI supporting Internet connectivity.
The 1990s were also known for the "Fieldbus Wars", where rival organizations competed
to define what would become the IEC fieldbus standard for digital communication with field
instrumentation instead of 4–20 milliamp analog communications. The first fieldbus
installations occurred in the 1990s. Towards the end of the decade, the technology began
to develop significant momentum, with the market consolidated around Ethernet I/P, Foundation
Fieldbus and Profibus PA for process automation applications. Some suppliers built new systems
from the ground up to maximize functionality with fieldbus, such as Rockwell PlantPAX System,
Honeywell with Experion & Plantscape SCADA systems, ABB with System 800xA, Emerson Process
Management with the Emerson Process Management DeltaV control system, Siemens with the SPPA-T3000
or Simatic PCS 7,Forbes Marshall with the Microcon+ control system and Azbil Corporation
with the Harmonas-DEO system. Fieldbus technics have been used to integrate machine, drives,
quality and condition monitoring applications to one DCS with Metso DNA system.
The impact of COTS, however, was most pronounced at the hardware layer. For years, the primary
business of DCS suppliers had been the supply of large amounts of hardware, particularly
I/O and controllers. The initial proliferation of DCSs required the installation of prodigious
amounts of this hardware, most of it manufactured from the bottom up by DCS suppliers. Standard
computer components from manufacturers such as Intel and Motorola, however, made it cost
prohibitive for DCS suppliers to continue making their own components, workstations,
and networking hardware. As the suppliers made the transition to COTS
components, they also discovered that the hardware market was shrinking fast. COTS not
only resulted in lower manufacturing costs for the supplier, but also steadily decreasing
prices for the end users, who were also becoming increasingly vocal over what they perceived
to be unduly high hardware costs. Some suppliers that were previously stronger in the PLC business,
such as Rockwell Automation and Siemens, were able to leverage their expertise in manufacturing
control hardware to enter the DCS marketplace with cost effective offerings, while the stabilityreliability
and functionality of these emerging systems are still improving. The traditional DCS suppliers
introduced new generation DCS System based on the latest Communication and IEC Standards,
which resulting in a trend of combining the traditional concepts/functionalities for PLC
and DCS into a one for all solution—named "Process Automation System". The gaps among
the various systems remain at the areas such as: the database integrity, pre-engineering
functionality, system maturity, communication transparency and reliability. While it is
expected the cost ratio is relatively the same, the reality of the automation business
is often operating strategically case by case. The current next evolution step is called
Collaborative Process Automation Systems. To compound the issue, suppliers were also
realizing that the hardware market was becoming saturated. The life cycle of hardware components
such as I/O and wiring is also typically in the range of 15 to over 20 years, making for
a challenging replacement market. Many of the older systems that were installed in the
1970s and 1980s are still in use today, and there is a considerable installed base of
systems in the market that are approaching the end of their useful life. Developed industrial
economies in North America, Europe, and Japan already had many thousands of DCSs installed,
and with few if any new plants being built, the market for new hardware was shifting rapidly
to smaller, albeit faster growing regions such as China, Latin America, and Eastern
Europe. Because of the shrinking hardware business,
suppliers began to make the challenging transition from a hardware-based business model to one
based on software and value-added services. It is a transition that is still being made
today. The applications portfolio offered by suppliers expanded considerably in the
'90s to include areas such as production management, model-based control, real-time optimization,
plant asset management, Real-time performance management tools, alarm management, and many
others. To obtain the true value from these applications, however, often requires a considerable
service content, which the suppliers also provide.
New age systems of 2010 onwards In the new world of distributed control system
following new technologies are emerging and taking roots :
1.>Wireless systems and protocols - Esp. ISA 100 and Wireless HART 2.>Remote transmissions
, logging and data historian 3.>Mobile interfaces and controls 4.>Embedded webservers
Increasingly and ironically distributed control systems are getting centralised at plant level
and are getting distributed in the ability to log in and access providing a superior
man machine interface esp. from remote access and portability standpoint.
As wireless protocols are getting refined by the day its is increasingly getting integrated
into DCS . Controllers of DCS are coming with embedded servers and provide on the go web
access. Most vendors have their HMI mobile ready both for android and IOS . With these
interfaces threat of breach of security hence danger to plant and process control are now
very real. See also
OSIsoft BNF Technology Inc.| ARIDES
Building Automation Direct Digital Control
SCADA PLC
Fieldbus First-out alarm
Midac Safety instrumented system,
Industrial control systems Industrial safety systems
Annunciator panel EPICS
TANGO References
^ "Introduction to Industrial Control Networks". IEEE Communications Surveys and Tutorials.
2012. ^ Stout, T. M. and Williams, T. J.. "Pioneering
Work in the Field of Computer Process Control". IEEE Annals of the History of Computing 17.
^ [1] CENTUM ^ a b [2] Metso DNA
^ [3] INFI 90 ^ [4] DCI-4000
^ [5] Foundation Fieldbus ^ [6] Foxboro I/A Series Distributed Control
System ^ ABB System 800xA
^ [7] Emerson Process Management ^ [8] SPPA-T3000
^ [9] Simatic PCS 7 ^ [10] Forbes Marshall
^ [11] Azbil Corporation External links
DCS Selection MBA research program with many Links
Example of DCS system: Mark VIe by General Electric
An even better example of DCS system: Control Design Platform by ICD
Proview is probably the first Open Source system for process control and automation
in the world. Open Source DCS and automation software
FreeDCS is another Open Source Distributed Control System