FUNDAMENTALS

OF

DISTRIBUTED CONTROL

SYSTEM(DCS)

ALOK SAIKIA (GAU-C-12/L-201)

JURI BARMAN (GAU-C-12/L-199)

CENTRAL INSTITUTE OF TECHNOLOGY (C.I.T.), KOKRAJHAR(A centrally funded institute under MHRD, Govt. Of India)

INTRODUCTION A Distributed Control System (DCS) refers to a control system usually of a manufacturing system,

process or any kind of dynamic system, in which the controller elements are not central in location

but are distributed throughout the system with each component sub-system controlled by one or

more controllers.

A DCS typically uses custom designed processor as controllers and uses both proprietary

interconnections and communication protocols for communication.

DCSs use multi-tasking operating systems, so user can download and run applications aside from

the real-time control functions and still do fractional-second control.

DCSs have user-friendly configuration tools, including structured English, control block libraries,

SFC (sequential function chart), and even RLL (relay ladder logic).

BASIC CONCEPT 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 transmit

instructions to the output instruments in the field.

Computer buses or electrical buses connect the processor and modules through multiplexer or de-

multiplexers. Buses also connect the distributed controllers with the central controller and finally

to the human-machine interface (HMI) or control consoles.

DCS allow centralized configuration from the operator or engineering console in the control room.

User can change programming offline, and download without restarting the system for the change

to be effective.

Digital control systems 

Centralize control system Distributed control system

Block diagram of a centralized control system

Block diagram of DCS

Block diagram contd.

From the block diagram,

PV= Process variable or input

MV= Manipulated variable or output

FCS= Field control station

OPS= Operator station

Basic components of DCS FCS (field control station):

It is used to control the process. All the instruments and interlocks created by software reside in the memory of the FCS. All the field instruments like transmitters and control valves are wired to the FCS. OPS (operator station):

It is used to monitor the process and to operate various instruments Communication bus:

It is used to communicate between the FCS and the OPS

Types of controller modeThere are four types of modes-

a) MAN(manual mode)

b) AUT(auto mode)

c) CAS (cascade mode)

d) PRD(primary direct mode)

Types of controller mode contd.

Manual Mode(MAN):

In manual mode, the MV pointer appears in red colour and the SV pointer appears

in yellow colour. MV of the instrument can be changed by increment/decrement keys or through data

entry box. The SV of the instrument can be changed either by set point change key or using ITEM

and DATA in the data entry box.

Auto Mode(AUT):

In auto mode, the SV pointer appears in red colour and the MV pointer appears in

yellow colour. SV of the instrument can be changed by increment/decrement keys or through data

entry box. The MV of the instrument cannot be changed by any method.

Cascade Mode(CAS)

In a cascade loop the output of the primary controller (MV1) goes as set point of the secondary controller (SV2). The output of the secondary controller (MV2) goes to the final control element. For a cascade loop, the primary controller can be in AUT or MAN mode, but the secondary controller has to be in CAS mode. In cascade mode both the SV and MV pointers appear in yellow colour. The SV and MV of the instrument cannot be changed.

Primary Direct Mode (PRD):

In PRD mode the output of the primary controller (MV1) directly goes to the final control

element. The secondary controller is bypassed and cannot be operated.

What is Human Interface Station(HIS)

It is one if the most important part of the DCS system. It is the point of interconnection between the field instrumentation and human being. From this station the startup and shutdown the operation and monitoring is controlled.

The HIS is provided with a number of useful functions that can be used with other general-purpose Windows applications.

a) Calling a general-purpose windows application while monitoring the plant

b) Calling the operation and monitoring window while using a general-purpose windows application

c) Display multiple windows always in the same format

Moreover, in order to improve operability, the following functions are prepared in HIS.

• Setup for using the shortcut keys

• Setup for using the context menus in graphic window

What is Engineering Station(ENG)

Engineering station is used to do the engineering builder for all the stations like HIS, FCS, CGW,

BCV etc. ENG is a PC loaded with engineering software.

The HIS can be loaded with engineering software so that it can be used as his as well as ENG.

• CGW:

Communication gateway unit used to communicate with supervisory computers.

• BCV:

Bus converter is used to link two domains.

 

Advantages

Control function is distributed among multiple CPUs (field control stations). Hence failure of one

FCS does not affect the entire plant.

 Redundancy is available at various levels.

Instruments and interlocks are created by software.

 Generation and modifications of the interlocks are very flexible and simple.

Information regarding the process is presented to the user in various formats.

Field wiring is considerably less.

Maintenance and troubleshooting becomes very easy.

Cost effective in the long run.