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DCS Basics for Engineers

A distributed control system (DCS) consists of controllers distributed throughout a system to control its components and subsystems. The controllers are connected by a communication network to monitor and control the overall system. A DCS has an operator station to collect and display process data, control stations containing control functions, and a communication system connecting everything. It allows individual components to be controlled even if other parts fail, provides redundancy, and reduces wiring needs, making DCS cost-effective for large systems.

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631 views5 pages

DCS Basics for Engineers

A distributed control system (DCS) consists of controllers distributed throughout a system to control its components and subsystems. The controllers are connected by a communication network to monitor and control the overall system. A DCS has an operator station to collect and display process data, control stations containing control functions, and a communication system connecting everything. It allows individual components to be controlled even if other parts fail, provides redundancy, and reduces wiring needs, making DCS cost-effective for large systems.

Uploaded by

joyal rathinam
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Distributed Control System (DCS) basics

by SivaranjithMarch 28, 201802973


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 Concept of Distributed Control System:


DCS refers to a control system usually of a manufacturing system, process or
any kind of dynamic system. In DCS controllers are not central in location, but
which are distributed throughout the system with each component sub-
system controlled by one or more controllers.
The entire system is connected by a communication network to control and
monitor the system.

The basic architecture of the distributed control system, as shown:

The DCS consists of an operator station, a control station, and a


communication system. The operator station collects data relating to the
process operation and can display and manipulate those data. The control
station contains control functions such as for the DDC.

Components of DCS:
The distributed control system (DCS) consists of the four interfaces. The
process interface is the interface between the distributed control system and
the plant (i.e., measurement sensors and final control elements).

Field Control Station:


The control stations receive measurement signals from sensors such as for
temperature, pressure, and flow rate, and perform control calculations in
accordance with the deviations from the setpoint values. Output signals are
then sent to the final control element to perform compensatory actions.

Human Machine Interface station:


The man-machine interface is the interface between the distributed control
system and the operator. It does central monitoring of the plant and permits
the operator to perform operations. The operator console is composed of a
powerful microprocessor, CRT, and keyboard. Many different operations can
be carried out by switching displays on
the CRT.

Control sub-interface:
The control sub-system interface connects the distributed control system to
other types of instruments such as the PLC (programmable logic controller) or
a composition analyzer to integrate plant operation.

Communication Bus:
Communication buses are used to communicate between HMI station and
control interface. It permits system build-up and software maintenance in the
distributed control system.

5 Level system in DCS


 

Configuration of the control station:


 
The control station consists mainly of a station control nest (SCN) and 1/0
nests. The SCN has a microprocessor and performs process control,
communication control, and duplex control. The 1/0 nests contain interface
cards for input and output. Signal conditioner nests and terminal boards can
be installed in the rear of the cabinet, as necessary.

Analog signals such as 4 to 20 rnA DC from the field transmitters and


thermocouple inputs are linearized and/or
normalized to 1 to 5 V DC by signal conditioner cards. The inputs are cabled
into a group of 8 or 16 points to the 1/0 card. These signals are converted to
digital values by the 1/0 cards.
The digital control output is converted to 4 to 20 rnA DC analog output signals
and sent to valves and other such devices via signal conditioners.

Advantages of DCS:
 Failure of one FCS deosn’t affect the entire plant
 Redundancy is avilable at various level
 Instruments and interlocks are created by software
 Field wiring is considerably less
 Cost effective in large
 

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