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SCADA

SCADA

(Supervisory Control And Data Acquisition) Systems are computer based
communications systems used by electric utilities to monitor and control the state of a
power distribution network.

SCADA may be called Human
-
Machine Interface (HMI) in Eur
ope. The term refers to a
large
-
scale, distributed measurement (and control) system. SCADA systems are used to
monitor or to control chemical, physical or transport processes.
________________________________________________________________________
Followin
g we describe the SCADA systems in terms of their architecture, their interface
to the process hardware, the functionality and the application development facilities they
provide.

SCADA systems have made substantial progress over the recent years in terms

of
functionality, scalability, performance and openness such that they are an alternative to in
house development even for very demanding and complex control systems.

What does SCADA mean? SCADA stands for Supervisory Control And Data
Acquisition. As the

name indicates, it is not a full control system, but rather focuses on
the supervisory level. As such, it is a purely software package that is positioned on top of
hardware to which it is interfaced, in general via Programmable Logic Controllers
(PLC's),
or other commercial hardware modules.

SCADA systems are used not only in industrial processes: e.g. steel making, power
generation (conventional and nuclear) and distribution, chemistry, but also in some
experimental facilities such as nuclear fusion. The

size of such plants range from a few
1000 to several 10 thousands input/output (I/O) channels. However, SCADA systems
evolve rapidly and are now penetrating the market of plants with a number of I/O
channels of several 100 thousands I/O's

SCADA systems u
sed to run on DOS, VMS and UNIX; in recent years all SCADA
vendors have moved to NT, Windows XP, Windows Server 2003 and some also to
Linux.

1. Architecture


This section describes the common features of the SCADA products.

HardwareArchitecture


One dist
inguishes two basic layers in a SCADA system: the "client layer" which caters
for the man machine interaction and the "data server layer" which handles most of the
process data control activities. The data servers communicate with devices in the field
thro
ugh process controllers. Process controllers, e.g. PLC's, are connected to the data
servers either directly or via networks or fieldbuses that are proprietary (e.g. Siemens

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H1), or non
-
proprietary (e.g. Profibus). Data servers are connected to each other a
nd to
client stations via an Ethernet LAN.

Software

Architecture

The products are multi
-
tasking and are based upon a real
-
time database (RTDB) located
in one or more servers. Servers are responsible for data acquisition and handling (e.g.
polling control
lers, alarm checking, calculations, logging and archiving) on a set of
parameters, typically those they are connected to.


Figure 1: Typical Hardware Architecture


Software Architecture

The products are multi
-
tasking and are based upon a real
-
time database (RTDB) located
in one or more servers. Servers are responsible for data acquisition and handling (e.g.
polling controllers, alarm checking, calculations, logging
and archiving) on a set of
parameters, typically those they are connected to.


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Figure 2: Generic Software Architecture

However, it is possible to have dedicated se
rvers for particular tasks, e.g. historian,
datalogger, alarm handler. The figure above shows a generic SCADA software
architecture.

Communications


Internal Communication:

Server
-
client and server
-
server communication is in general on a publish
-
subscribe

and
event
-
driven basis and uses a TCP/IP protocol, i.e., a client application subscribes to a
parameter which is owned by a particular server application and only changes to that
parameter are then communicated to the client application.

Access to Device
s

The data servers poll the controllers at a user defined polling rate. The polling rate may
be different for different parameters. The controllers pass the requested parameters to the
data servers. Time stamping of the process parameters is typically perf
ormed in the
controllers and this time
-
stamp is taken over by the data server. If the controller and
communication protocol used support unsolicited data transfer then the products will
support this too.


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The products provide communication drivers for most

of the common PLC's and widely
used field
-
buses, e.g., Modbus. Some of the drivers are based on third party products
(e.g., Applicom cards) and therefore have additional cost associated with them. VME on
the other hand is generally not supported.

A singl
e data server can support multiple communications protocols: it can generally
support as many such protocols as it has slots for interface cards.

The effort required to develop new drivers is typically in the range of 2
-
6 weeks
depending on the complexity

and similarity with existing drivers, and a driver
development toolkit is provided for this.

Interfacing

Application Interfaces / Openness:

The provision of OPC client functionality for SCADA to access devices in an open and
standard manner is developing
. There still seems to be a lack of devices/controllers,
which provide OPC server software, but this improves rapidly as most of the producers of
controllers are actively involved in the development of this standard.

The products also provide an Open Data

Base Connectivity (ODBC) interface to the data
in the archive/logs, but not to the configuration database, an ASCII import/export facility
for configuration data, a library of APIs supporting C, C++, and Visual Basic (VB) to
access data in the RTDB, logs
and archive. The API often does not provide access to the
product's internal features such as alarm handling, reporting, trending, etc.

The PC products provide support for the Microsoft standards such as Dynamic Data
Exchange (DDE) which allows e.g. to vi
sualize data dynamically in an EXCEL
spreadsheet, Dynamic Link Library (DLL) and Object Linking and Embedding (OLE).

Database

The configuration data are stored in a database that is logically centralised but physically
distributed and that is generally o
f a proprietary format.

For performance reasons, the RTDB resides in the memory of the servers and is also of
proprietary format.

The archive and logging format is usually also proprietary for performance reasons, but
some products do support logging to
a Relational Data Base Management System
(RDBMS) at a slower rate either directly or via an ODBC interface.

Scalability


Scalability is understood as the possibility to extend the SCADA based control system by
adding more process variables, more specializ
ed servers (e.g. for alarm handling) or more
clients. The products achieve scalability by having multiple data servers connected to

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multiple controllers. Each data server has its own configuration database and RTDB and
is responsible for the handling of a
sub
-
set of the process variables (acquisition, alarm
handling, archiving).

Redundancy

The products often have built in software redundancy at a server level,
which is normally transparent to the user. Many of the products also provide more
complete redund
ancy solutions if required.

2. Functionality


Access Control

Users are allocated to groups, which have defined read/write access privileges to the
process parameters in the system and often also to specific product functionality.

MMI

The products suppor
t multiple screens, which can contain combinations of synoptic
diagrams and text.

They also support the concept of a "generic" graphical object with links to process
variables. These objects can be "dragged and dropped" from a library and included into a
synoptic diagram.

Most of the SCADA products that were evaluated decompose the process in "atomic"
parameters (e.g. a power supply current, its maximum value, its on/off status, etc.) to
which a Tag
-
name is associated. The Tag
-
names used to link graphical

objects to devices
can be edited as required. The products include a library of standard graphical symbols,
many of which would however not be applicable to the type of applications encountered
in the experimental physics community.

Standard windows edit
ing facilities are provided: zooming, re
-
sizing, scrolling... On
-
line
configuration and customization of the MMI is possible for users with the appropriate
privileges. Links can be created between display pages to navigate from one view to
another.

Trendi
ng

The products all provide trending facilities and one can summarize the common
capabilities as follows:

● the parameters to be trended in a specific chart can be predefined or defined on
-
line

● a chart may contain more than 8 trended parameters or pens

and an unlimited number
of charts can be displayed (restricted only by the readability)

● real
-
time and historical trending are possible, although generally not in the same chart

● historical trending is possible for any archived parameter

● zooming and

scrolling functions are provided

● parameter values at the cursor position can be displayed


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The trending feature is either provided as a separate module or as a graphical object
(ActiveX), which can then be embedded into a synoptic display. XY and other
statistical
analysis plots are generally not provided.

Alarm Handling

Alarm handling is based on limit and status checking and performed in the data servers.
More complicated expressions (using arithmetic or logical expressions) can be developed
by creat
ing derived parameters on which status or limit checking is then performed. The
alarms are logically handled centrally, i.e., the information only exists in one place and
all users see the same status (e.g., the acknowledgement), and multiple alarm priorit
y
levels (in general many more than 3 such levels) are supported.

It is generally possible to group alarms and to handle these as an entity (typically filtering
on group or acknowledgement of all alarms in a group). Furthermore, it is possible to
suppress

alarms either individually or as a complete group. The filtering of alarms seen
on the alarm page or when viewing the alarm log is also possible at least on priority, time
and group. However, relationships between alarms cannot generally be defined in a
s
traightforward manner. E
-
mails can be generated or predefined actions automatically
executed in response to alarm conditions.

Logging/Archiving

The terms logging and archiving are often used to describe the same facility. However,
logging can be thought
of as medium
-
term storage of data on disk, whereas archiving is
long
-
term storage of data either on disk or on another permanent storage medium.
Logging is typically performed on a cyclic basis, i.e., once a certain file size, time period
or number of poin
ts is reached the data is overwritten. Logging of data can be performed
at a set frequency, or only initiated if the value changes or when a specific predefined
event occurs. Logged data can be transferred to an archive once the log is full. The
logged dat
a is time
-
stamped and can be filtered when viewed by a user. The logging of
user actions is in general performed together with either a user ID or station ID. There is
often also a VCR facility to play back archived data.

Report Generation

One can produce

reports using SQL type queries to the archive, RTDB or logs. Although
it is sometimes possible to embed EXCEL charts in the report, a "cut and paste"
capability is in general not provided. Facilities exist to be able to automatically generate,
print and a
rchive reports.

Automation

The majority of the products allow actions to be automatically triggered by events. A
scripting language provided by the SCADA products allows these actions to be defined.
In general, one can load a particular display, send an
Email, run a user defined
application or script and write to the RTDB.

The concept of recipes is supported, whereby a particular system configuration can be
saved to a file and then re
-
loaded at a later date.


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Sequencing is also supported whereby, as the
name indicates, it is possible to execute a
more complex sequence of actions on one or more devices. Sequences may also react to
external events.

3. Application Development


Configuration

The development of the applications is typically done in two stage
s. First the process
parameters and associated information (e.g. relating to alarm conditions) are defined
through some sort of parameter definition template and then the graphics, including
trending and alarm displays are developed, and linked where appro
priate to the process
parameters. The products also provide an ASCII Export/Import facility for the
configuration data (parameter definitions), which enables large numbers of parameters to
be configured in a more efficient manner using an external editor s
uch as Excel and then
importing the data into the configuration database.

However, many of the PC tools now have a Windows Explorer type development studio.
The developer then works with a number of folders, which each contains a different
aspect of the c
onfiguration, including the graphics.

The facilities provided by the products for configuring very large numbers of parameters
are not very strong. However, this has not really been an issue so far for most of the
products to
-
date, as large applications a
re typically about 50k I/O points and database
population from within an ASCII editor such as Excel is still a workable option.

Online modifications to the configuration database and the graphics are generally
possible with the appropriate level of privil
eges.

Development Tools

The following development tools are provide
d as standard:

● Graphics Editor, with standard drawing facilities including freehand, lines, squares
circles, etc. It is possible to import pictures in many formats as well as using predefined
symbols including e.g. trending charts, etc. A library of gene
ric symbols is provided that
can be linked dynamically to variables and animated as they change. It is also possible to
create links between views so as to ease navigation at run
-
time.

● Database Configuration Tool (usually through parameter templates). I
t is in general
possible to export data in ASCII files so as to be edited through an ASCII editor or Excel.

● Scripting Language

● Application Program Interface (API) supporting C, C++, VB

● Driver Development Toolkit to develop drivers for hardware that

is not supported by
the SCADA product.

Object Handling

The products in general have the concept of graphical object classes, which support
inheritance. In addition, some of the products have the concept of an object within the
configuration database. In g
eneral the products do not handle objects, but rather handle

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individual parameters, e.g., alarms are defined for parameters, logging is performed on
parameters, and control actions are performed on parameters. The support of objects is
therefore fairly sup
erficial.

4. Evolution


SCADA vendors release one major version and one to two additional minor versions
once per year. These products evolve thus very rapidly so as to take advantage of new
market opportunities, to meet new requirements of their customer
s and to take advantage
of new technologies.

As was already mentioned, most of the SCADA products that were evaluated decompose
the process in "atomic" parameters to which a Tag
-
name is associated. This is impractical
in the case of very large processes w
hen very large sets of Tags need to be configured. As
the industrial applications are increasing in size, new SCADA versions are now being
designed to handle devices and even entire systems as full entities (classes) that
encapsulate all their specific att
ributes and functionality. In addition, they will also
support multi
-
team development.

As far as new technologies are concerned, the SCADA products are now adopting:

● Web technology, ActiveX, Java, etc.

● OPC as a means for communicating internally between the client and server modules.
It should thus be possible to connect OPC compliant t
hird party modules to that SCADA
product.

5. Engineering


Whilst one should rightly anticipate significant development and maintenance savings by
adopting a SCADA product for the implementation of a control system, it does not mean
a "no effort" operation.

The need for proper engineering can not be sufficiently
emphasized to reduce development effort and to reach a system that complies with the
requirements, that is economical in development and maintenance and that is reliable and
robust. Examples of engin
eering activities specific to the use of a SCADA system are the
definition of:

● A library of objects (PLC, device, subsystem) complete with standard object behavior
(script, sequences, ...), graphical interface and associated scripts for animation

● Templ
ates for different types of "panels", e.g. alarms

● Instructions on how to control e.g. a device ...

● A mechanism to prevent conflicting controls (if not provided with the SCADA)

● Alarm levels, behavior to be adopted in case of specific alarms, ...

Ivens
ys
-

Wonderware / Intouch


InTouch® 9.0 software with SmartSymbols and the IOSetRemoteReferences script
function enables users to quickly and easily create and deploy graphical representations

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of real
-
time industrial process applications that connect to In
Touch tag servers,
ArchestrA® Object Templates in Wonderware's Industrial Application Server and I/O
Servers.

With SmartSymbols, users can very easily create graphic templates that can be used
throughout the entire application. Users can create a graphica
l object once, attach
animations and then save that object as a SmartSymbol. Users can also create standard
libraries of SmartSymbols that adhere to their company's standards for color and
animation, resulting in graphics that conform to existing practices

without requiring a
great deal of administration and management. These libraries of SmartSymbols can be
exported and imported into other InTouch applications resulting in standards for graphics
that can be easily implemented throughout an entire organizat
ion. Developing entire
InTouch HMI applications becomes as simple as choosing the SmartSymbol graphic
from the library manager, selecting the instance reference and dropping it into a window.

When testing applications or modifying graphical objects, users

only need to edit the
SmartSymbol graphic template and all instances throughout the application will be
automatically updated with the new information, resulting in tremendous time savings
and a significant reduction in potential errors.

In addition, the

IOSetRemoteReferences script function enables users to create graphical
faceplates, which can be quickly modified at runtime. Faceplates can be created to model
devices and their controls used throughout the application such as valves, pumps and
motors. T
o leverage the IOSetRemoteReferences script function, a user would first create
a SmartSymbol graphic template and then associate it with tags using a remote style
reference. At runtime, whenever a particular condition occurs or a device such as a push
of
a button is activated, the IOSetRemoteReferences function updates all of the data
references. This update is very fast because all of the data sources in the window are
updated using one line of script.


Wonderware
-

Intouch

GE Fanuc
-

Intellution / iFIX


Proficy HMI/SCADA
-

iFIX is a powerful Client/Server based HMI/SCADA solution
that provides process visualization, data acquisition and supervisory

control over
manufacturing and production processes. Proficy HMI/SCADA
-

iFIX gives Operators
and Process Engineers the power and security to precisely monitor and control every

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aspect of their process, equipment and resources. The result is a faster resp
onse to
production issues, with improved quality, reduced waste, faster time
-
to
-
market and
increased profitability.

Powerful Distributed Client/Server Architecture Collects, processes and distributes real
-
time data with unparalleled flexibility and scalab
ility. The Proficy HMI/SCADA
-

iFIX
architecture enables users to leverage multiple clients, including iClient TS
-

a solution
that leverages Microsoft Terminal Server technology to seamlessly extend the reach of
your HMI/SCADA applications.

Faster system

development and deployment The Intellution WorkSpace delivers point
-
and
-
click simplicity to application development. Through the use of powerful yet easy
-
to
-
use Wizards, Proficy HMI/SCADA
-

iFIX dramatically accelerates the development
process. In additio
n, Intellution's Animation Experts drive internal third
-
party ActiveX
control without VBA Programming.

Simplified application integration Through Proficy HMI/SCADA
-

iFIX's patented
Secure Containment technology, you can fully leverage third
-
party applica
tions within
the Proficy HMI/SCADA
-

iFIX environment... and do so without compromising your
system's reliability.

Enhanced security and accountability Proficy HMI/SCADA
-

iFIX boasts powerful new
security and eSignature capabilities, designed to enable a
ccess restriction at a very
granular level, as well as deliver a vehicle for capturing complete audit trail information
-

outstanding functionality for businesses in the regulated industries or for any company
who simply wants to enhance security.


DCS or PLC? What is the difference?

You must automate a process, but you can't decide between a DCS and a PLC. Are these
systems really all that different?

The answers depend on a slew of other questions.

The
Programmable Logic Controller

(PLC) is king of machine control while the
Distributed Control System

(DCS) dominates process control. If you manufacture
plastic widgets, you speak PLC. If you produce ch
emicals, you speak DCS.

Today, the two technologies share kingdoms as the functional lines between them
continue to blur. We now use each where the other used to rule. However, PLCs still

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dominate high
-
speed machine control, and DCSs prevail in complex co
ntinuous
processes.

The early DCS looked dramatically different from the early PLC. Initially, the DCS
performed the control functions of the analog panel instruments it replaced, and its
interface mimicked their panel displays. DCSs then gained sequence
logic capabilities to
control batch processes as well as continuous ones. DCSs performed hundreds of analog
measurements and controlled dozens of analog outputs, using multi
-
variable Proportional
Integral Derivative (PID) control. With the same 8
-
bit micro
processor technology that
gave rise to the DCS, PLCs began replacing conventional relay/solid
-
state logic in
machine control. PLCs dealt with contact input/output (I/O) and started/stopped motors
by performing Boolean logic calculations.

The big change in

DCS over the past 20 years is its move from proprietary hardware to
the personal computer (PC) and standard LAN technologies. With each advance in PC
power, DCSs have moved up in power. PCs gave us speedy, responsive, multi
-
media,
windowed, operator
-
proce
ss interfaces (OPI). Relational databases and spreadsheet
software enhance the ability of DCSs to store and manipulate data. Artificial intelligence
(AI) technology gives us "smart" alarming. Today's DCS architecturally looks much like
the DCS of 20 years
ago, but tomorrow's DCS may control through networked "smart"
devices
-
with no I/O hardware of its own.

Most DCSs offer redundant controllers, networks, and I/Os. Most give you "built
-
in"
redundancy and diagnostic features, with no need for user
-
written lo
gic.

DCSs allow centralized configuration from the operator or engineering console in the
control room. You can change programming offline, and download without restarting the
system for the change to be effective.

DCSs allow inter
-
controller communicati
ons. You can do data exchange in most DCS
systems ad hoc (no need for predefined data point lists). You access data by tag name,
regardless of hardware or location.

DCSs use multi
-
tasking operating systems, so you can download and run applications
aside f
rom the real
-
time control functions and still do fractional
-
second control. DCSs
now come in "micro" systems, to price
-
compete with PLCs
-
but with full DCS features
and capabilities.

The typical DCS has integrated diagnostics and standard display templates

that
automatically extend/update when your database changes. This database is central to the
system
-
you don't have different databases sitting in the controllers.

DCSs have user
-
friendly configuration tools, including structured English, control block
li
braries, SFC (sequential function chart), and even RLL (relay ladder logic).


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Most DCSs allow graphical configuration, provide online diagnostics, and are self
-
documenting. Most provide for user
-
defined control blocks or customized strategies. The
controll
ers execute control strategies as independent tasks; thus, making changes to part
of the control logic has no impact on the rest.

An important difference between DCSs and PLCs is how vendors market them. DCS
vendors typically sell a complete, working, int
egrated, and tested system; offering full
application implementation. They offer many services: training, installation, field service,
and integration with your Information Technology (IT) systems. A DCS vendor provides
a server with a relational database,

a LAN with PCs for office automation, networking
support and integration of third
-
party applications and systems. The DCS vendor tries to
be your "one
-
stop shop." The PLC is more of a "do
-
it
-
yourself" device, which is
sometimes simpler to execute.


Programmable Logic Controllers. When PLCs were solely replacements for hard
-
wired
relays, they had only digital I/O, with no operator interface or communications. Simpl
e
operator interfaces appeared, then evolved into increasingly complex interfaces as PLCs
worked with increasingly complex automation problems. We went from a panel of
buttons and I/O
-
driven lamps to PLC full
-
color customized graphic displays that run on
S
CADA software over a network.

PLCs now have many DCS
-
like control functions (e.g., PID algorithms) and analog I/O.
They've moved past their birthplace: the digital world (switch and binary sensor inputs
and output contacts to run motors and trigger soleno
ids).

PLCs are fast: They run an input
-
compute
-
output cycle in milliseconds. On the other
hand, DCSs offer fractional second (1/2 to 1/10) control cycles. However, some DCSs
provide interrupt/event
-
triggered logic for high
-
speed applications.

PLCs are si
mple, rugged computers with minimal peripherals and simple OSs. While
increasing reliability, PLC simplicity is not conducive to redundancy. Thus, fully
redundant ("hot," automatic, bumpless) variations of PLCs, with their added hardware

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and software, some
times suffer from a reduction in their reliability
-
a characteristic PLCs
are famous for.

Data exchange typically requires you to preassign data registers and hard code their
addresses into the logic. If you add registers or need to reassign data, you typi
cally have
to deal manually with the Domino Effect.

Typical PLC Relay Ladder Logic (RLL) languages include function blocks that can
perform complex control and math functions (e.g., PID algorithms). Complex multi
-
loop
control functions (e.g., cascade mana
gement and loop initialization) are not typical. For
functions too messy to implement in RLL, most PLCs provide a function block that calls
a user
-
written program (usually in BASIC or C).

PLCs typically operate as "state" machines: They read all inputs, e
xecute through the
logic, and then drive the outputs. The user
-
written logic is typically one big RLL
program, which means you may have to take the whole PLC off
-
line to make a change of
any size. You also run into database synchronization problems because

of the separation
of PLCs and the Man Machine Interface (MMI) software packages, as opposed to the
central databases of DCSs.

A PLC will run in a stand
-
alone configuration. A DCS controller normally expects an
operator interface and communications, so it

can send alarms, messages, trend updates,
and display updates.

Many PLC installations use interface software from third
-
party vendors for improved
graphics and various levels of alarming, trending, and reporting. The PLC and MMI
software normally interac
t by sitting on the network and using the register exchange
mechanism to get data from and to the various PLCs. This type of communication
presumes you have preassigned data registers and can fetch data on an absolute address
basis. This can lead to data p
rocessing errors (e.g., from the wrong input) you won't
encounter with the central database of a DCS.

Some PLCs use proprietary networks, and others can use LANs. Either way, the
communication functions are the same
-
fetch and put registers. This can resul
t in
bottlenecking and timing problems if too many PCs try communicating with too many
PLCs over a network.

A PLC may have a third
-
party package for operator interfaces, LAN interface to PCs and
peripherals, PLC data highway or bus, redundant controllers
with local and distributed
I/O, local MMI and local programming capability. The PLC would have redundant media
support, but not the redundant communication hardware or I/O bus hardware you'd find
in a DCS. A PLC would have preprogrammed I/O cards for speci
fic signal types and
ranges.

Today, the decision between PLC and DCS often depends on business issues rather than
technical features. Questions to consider are those involving:


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14

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The internal expertise to execute the project, Level of support available fro
m a
vendor/integrator, Long
-
term maintainability, and Life
-
cycle costs.

PLCs and DCSs overlap in their features, but also have distinct strengths and weaknesses.
When deciding between the two, know who will deliver and support your system, and
how they wi
ll do it.

DCS


DCS

(Distributed Control System) is a control system in which the components that
make up the system are distributed geographically and functionally around the process
areas. The main function of the DCS is to receive inputs from field moun
ted instruments
(Flow, Pressure, Level, Temperature, Analysis etc), process the information in a
predetermined program and provide control signal to final control elements, generate
reports etc.

Thus DCS is a control system which collects the data from th
e field and decides what to
do with them. Data from the field can either be stored for future reference, used for
simple process control, use in conjunction with data from another part of the plant for
advanced control strategies.

________________________
________________________________________________


What must be in the DCS for it to be able to do so much?


Operator Console

These are like the monitors of our computers. They provide us with the feedback of what
they are doing in the plant as well as the
command we issue to the control system. These
are also the places where operators issue commands to the field instruments.

Engineering Station

These are stations for engineers to configure the system and also to implement control
algorithms.

History Module

This is like the harddisk of our PCs. They store the configurations of the DCS as well as
the configurations of all the points in the plant. They also store the graphic files that are
shown in the console and in most systems these days they are able to st
ore some plant
operating data.

Data Historian

These are usually extra pieces of software that are dedicated to store process variables,
set points and output values. They are usually of higher scanning rates than that available
in the history module.


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Contr
ol Modules

These are like the brains of the DCS. Specially customized blocks are found here. These
are customized to do control functions like PID control, ratio control, simple arithmetic
and dynamic compensation. These days, advanced control features can

also be found in
them.

I/O

These manage the input and output of the DCS. Input and output can be digital or
analogues. Digital I/Os are those like on/off, start/stop signals. Most of the process
measurements and controller outputs are considered analogue.

These are the points where
the field instruments are hard
-
wired to.

All above mentioned elements are connected by using a network, nowadays very often
used is Ethernet.


The practical and technological boundaries between a Distributed Control System DCS,
Programmable Logic Controller PLC and Personal Computer PC control are blurrin
g.
Systems traditionally associated with process control are being used in discrete
applications. Likewise, traditionally discrete solutions are used increasingly in both batch
and continuous process control.

Today's control hardware are constructed from
many of the same standard industry
components such as Intel processors. Therefore the only real difference between control
systems is at the software level.

________________________________________________________________________


ABB / Industrial IT
-

Ad
vant Master DCS

Advant OCS (Open Control System) is an ABB solution for operators to improve their
manufacturing productivity and achieve sustainable competitive advantages.

In 1992, based on the success of the Master systems in the 80's, the Master syste
m began
its evolution to Advant OCS. This evolution introduced high capacity controllers and I/O
with an improved redundancy scheme. Also included were modern UNIX workstations,
and in 1996 S800 I/O was added providing modular flexible remote I/O.


-

16

-

In 2000
, Advant OCS with Master Software began its next step in the evolution process
with the introduction of Industrial IT enabled products. ABB's commitment to protecting
your investment continues with these enhancements by providing connectivity to our
latest

control offering.

A versatile and complete range of process I/O systems within the Advant family enables
optimal user configurations:

S100I/O
-

A rack
-
based I/O system for AC400 controllers S600I/O
-

A rack
-
based I/O
system for AC100 controllers S800I/O

-

A highly modularized and flexible I/O
-
system

Numerous characteristics and functions facilitate and improve operation, monitoring, and
reengineering of each process in a company. 800xA Operations (Process Portal) and the
proven AdvaCommand for Unix solu
tion (based on HP
-
UX) are available as an operator
station for Advant OCS with Master Software.

The intuitive operator software provides consistent access and interaction with data from
multiple control and I/O to plant and enterprise information.


ABB Advant Master Control Systems

Honeywell Experion™ Process Knowledge System (PKS)

Experion is Honeywell's unified system for process, business, and asset management t
hat
helps industrial manufacturers increase their profitability and productivity.

Experion takes customers well beyond Distributed Control System (DCS) functionality
with an advanced automation platform solution and innovative application integration to
i
mprove business performance and peace of mind. And there's no need to worry about
upgrading from TDC 2000®/TDC 3000® or TotalPlant® Solution (TPS).

The unique, patent pending design of Series C combines sleek styling and function to
provide process I/O wi
th reduced footprint, easier installation and maintenance, and
longer life. The Series C form factor benefits extend to multiple modules, such as the
Series C C300 Controller, the Fieldbus Interface Module, the Control Firewall, and
HART analog modules.


-

17

-

T
he Control Execution Environment (CEE) is the common core software used in the
various controllers supported by ExperionTM. This includes the C200 Process Controller,
the C300 Process Controller, the Application Control Environment (ACE) and the C200
Simul
ation Environment (SIM
-
C200). The CEE provides an execution and scheduling
environment where control strategies are configured from a rich set of standard and
optional function blocks using a single builder tool, Control Builder.

Function blocks are group
ed and wired together in a container to perform a specific
control function such as a valve control strategy. The Control Execution Environment
(CEE) supports two types of containers: the Control Module in which continuous and
discrete controls are combine
d; and an SCM, which is used for sequence control.
Function blocks support the complete control application range, such as continuous,
discrete and batch control.


Honeywell
-

Experion PKS
________________________________________________________________________


Emerson Process Management / DeltaV

DeltaV is the creation of Emerson Process Management's technological innovators, who
worked in an off
-
site "out
-
of
-
the
-
box" think tank to build an automation system that
could integrate and leverage today's digital world and cutting
-
edge technological
innovations to make a value step
-
change in the process industries.

The name DeltaV is derived from the engineering equ
ation for acceleration: dv/dt, the
change in velocity over the change in time. The DeltaV system makes planning,
engineering, installing, commissioning, training, operating, and maintaining your process
EASY, which accelerates your success in improving you
r plant performance.

The DeltaV system scales the complete range of applications from an isolated process
area to a complete plant
-
wide automation system. Whether you need tens of I/O or tens of
thousands of I/O
-
any size you want! The DeltaV system provid
es all the tools to manage
your process easier than ever before.

The complete family of controllers is available to power your most advanced control
strategies. Full controller and power supply redundancy is available for your mission
-

-

18

-

critical application
s. The controller and I/O sub
-
system is rated for Class I, Division 2
and Zone 2 environments to reduce your installation costs.

DeltaV workstations are based on the latest Intel
-
based microprocessors running the
Microsoft Windows XP /Windows 2003 operati
ng system. A complete range of
applications is provided to cover system configuration, operator interface, engineering,
maintenance, and integration functions.

The DeltaV control network

a high
-
speed Ethernet LAN

provides system
communications and connect
s the various system nodes. The control network can be fully
redundant. DeltaV remote services extend the operations, engineering, and diagnostic
applications across your enterprise network.

Unlike PLC/HMI solutions, the completely integrated DeltaV syste
m features a single
database that oordinates all configuration activities. System configuration is globally
distributed in the run
-
time environment.



Emerson
-

DeltaV

Hybrid Systems





DISTRIBUTED CONTROL SYSTEMS

Introduction

Generally, the concept of automatic control includes accomplishing two major
operations; the transmission of signals (information flow) back and forth and the
calculation of control actio
ns (decision making). Carrying out these operations in real
plant requires a set of hardware and instrumentation that serve as the platform for these
tasks. Distributed control system (DCS) is the most modern control platform. It stands as
the infrastructu
re not only for all advanced control strategies but also for the lowliest
control system. The idea of control infrastructure is old. The next section discusses how
the control platform progressed through time to follow the advancement in control
algorithms

and instrumentation technologies.



1. Historical Review

To fully appreciate and select the current status of affairs in industrial practice it is of
interest to understand the historical perspective on the evolution of control systems
implementation phi
losophy and hardware elements. The evolution concerns the heart of
any control system which is how information flow and decision making advanced.


-

19

-


1. Pneumatic Implementation: In the early implementation of automatic control systems,
information flow was
accomplished by pneumatic transmission, and computation was
done by mechanical devices using bellows, spring etc. The pneumatic controller has high
margin for safety since they are explosion proof. However, There are two fundamental
problems associated wit
h pneumatic implementation:



Transmission
: the signals transmitted pneumatically (via air pressure) are slow
responding and susceptible to interference.


Calculation
: Mechanical computation devices must be relatively simple and tend to
wear out quickl
y.


2. Electron analog implementation:

Electrons are used as the medium of transmission
in his type of implementation mode. Computation devices are still the same as before.
Electrical signals to pressure signals converter (E/P transducers) and vice verse

(P/E
transducers) are used to communicate between the mechanical devices and electron flow.
The primary problems associated with electronic analog implementation are:



Transmission
: analog signals are susceptible to contamination from stray fields, and

signal quality tends to degrade over long transmission line.


Calculation
: the type of computations possible with electronic analog devices is still
limited.

Chemical Engineering Department King Saud University, 2002 Process Control in the
Chemical In
dustries 133


3. Digital Implementation:

the transmission medium is still electron, but the signals are
transmitted as binary numbers. Such digital signals are far less sensitive to noise. The
computational devices are digital computers. Digital computers

are more flexible because
they are programmable. They are more versatile because there is virtually no limitation to
the complexity of the computations it can carry out. Moreover, it is possible to carry out
computation with a single computing device, or
with a network of such devices.


Many field sensors naturally produce analog voltage or current signals. For this reason
transducers that convert analog signals to digital signals (A/D) and vice verse (D/A) are
used as interface between the analog and dig
ital elements of the modern control system.
With the development of digital implementation systems, which DCS are based on, it is
possible to implement many sophisticated control strategies on a very fast timescale.

2. Modes of Computer control

Computer
control is usually carried out in two modes: supervisory control or direct
digital control. Both are shown in Figure 1. Supervisory control involves resetting the set
point for a local controller according to some computer calculation. Direct digital contr
ol,
by contrast, requires that all control actions be carried out by the digital computer. Both
modes are in wide use in industrial applications, and both allow incorporating modern
control technologies. Measurements are transmitted to computer and control

signals are
sent from computer to control valves at specific time interval known as sampling time.
The latter should be chosen with care.


-

20

-


Figure 1: Computer control modes.


4. Computer Control Networks

Chemical Engineering Department King Saud Univer
sity, 2002 Process Control in the
Chemical Industries 134

The computer control network performs a wide variety of tasks: data acquisition,
servicing of video display units in various laboratories and control rooms, data logging
from analytical laboratorie
s, control of plant processes or pilot plant, etc. The computer
network can be as simple as an array of inexpensive PC's or it could be a large
commercial distributed control system (DCS).


4.1 Small Computer Network

In small processes such as laboratory

prototype or pilot plants, the number of control
loops is relatively small. An inexpensive and straightforward way to deal with the
systems is to configure a network of personal computers for data acquisition and control.
An example configuration of a PC
network control system is depicted in Figure 2. The
network consists of a main computer linked directly to the process in two
-
way channels.
Other local computers are linked to the main computer and are also connected to the
process through one
-
way or two
-
w
ay links. Some of these local computers can be
interconnected. Each of the local computers has a video display and a specific function.
For example, some local computers are dedicated for data acquisition only, some for
local control only and some other fo
r both data acquisition and local control. The main
computer could have a multiple displays.

All computers operate with a multitasking operating system. They would be normally
configured with local memory, local disk storage, and often have shared disk st
orage with
a server.


Figure 2: PC network


4.2 Programmable Logic Controllers


Chemical Engineering Department King Saud University, 2002 Process Control in the
Chemical Industries 135

Programmable logic controller (PLC) is another type of digital te
chnology used in
process control. It is exclusively specialized for non
-
continuous systems such as batch
processes or that contains equipment or control elements that operate discontinuously. It
can also be used for many instants where interlocks are requi
red; for example, a flow
control loop cannot be actuated unless a pump has been turned on. Similarly, during
startup or shutdown of continuous processes many elements must be correctly sequenced;
that is, upstream flows and levels must be established befor
e downstream pumps can be
turned on.

The PLC concept is based on designing a sequence of logical decisions to implement the
control for the above mentioned cases. Such a system uses a special purpose computer
called programmable logic controllers because
the computer is programmed to execute
the desired Boolean logic and to implement the desired sequencing. In this case, the

-

21

-

inputs to the computer are a set of relay contacts representing the state of various process
elements. Various operator inputs are al
so provided. The outputs from the computer are a
set of relays energized (activated) by the computer that can turn a pump on or off,
activate lights on a display panel, operate solenoid valve, and so on.

PLCs can handle thousands of digital I/O and hundre
ds of analog I/O and continuous PID
control. PLC has many features besides the digital system capabilities. However, PLC
lacks the flexibility for expansion and reconfiguration. The operator interface in PLC
systems is also limited. Moreover, programming P
LC by a higher
-
level languages and/or
capability of implementing advanced control algorithms is also limited.

PLCs are not typical in a traditional process plant, but there some operations, such as
sequencing, and interlock operations, that can use the po
werful capabilities of a PLC.
They are also quite frequently a cost
-
effective alternative to DCSs (discussed next) where
sophisticated process control strategies are not needed. Nevertheless, PLCs and DCSs can
be combined in a hybrid system where PLC conne
cted through link to a controller, or
connected directly to network.


4.3 Commercial Distributed Control Systems

In more complex pilot plants and full
-
scale plants, the control loops are of the order of
hundreds. For such large processes, the commercial
distributed control system is more
appropriate. There are many vendors who provide these DCS systems such as Baily,
Foxboro, Honeywell, Rosemont, Yokogawa, etc. In the following only an overview of
the role of DCS is outlined.

Conceptually, the DCS is sim
ilar to the simple PC network. However, there are some
differences. First, the hardware and software of the DCS is made more flexible, i.e. easy
to modify and configure, and to be able to handle a large number of loops. Secondly, the
modern DCS are equippe
d with optimization, high
-
performance model
-
building and
control software as options. Therefore, an imaginative engineer who has theoretical
background on modern control systems can quickly configure the DCS network to
implement high performance controller
s.

A schematic of the DCS network is shown in figure 3. Basically, various parts of the
plant processes and several parts of the DCS network elements are connected to
Chemical Engineering Department King Saud University, 2002 Process Control in the
Chemi
cal Industries 136

each others via the data highway (fieldbus). Although figure 3 shows one data highway,
in practice there could be several levels of data highways. A large number of local data
acquisition, video display and computers can be found distri
buted around the plant. They
all communicate to each others through the data highway. These distributed elements
may vary in their responsibilities. For example, those closest to the process handle high
raw data traffic to the local computers while those f
arther away from the process deal
only with processed data but for a wider audience.

The data highway is thus the backbone for the DCS system. It provides information to the
multi
-
displays on various operator control panels sends new data and retrieve his
torical
data from archival storage, and serves as a data link between the main control computer
and other parts of the network.

On the top of the hierarchy, a supervisory (host) computer is set. The host computer is
responsible for performing many higher
level functions. These could include

-

22

-

optimization of the process operation over varying time horizons (days, weeks, or
months), carrying out special control procedure such as plant start up or product grade
transition, and providing feedback on economic pe
rformance.


Figure 3: The elements of a commercial distributed control system network


A DCS is then a powerful tool for any large commercial plant. The engineer or operator
can immediately utilize such a system to:
Chemical Engineering Department King
Saud University, 2002 Process Control in the
Chemical Industries 137


• Access a large amount of current information from the data highway.

• See trends of past process conditions by calling archival data storage.

• Readily install new on
-
line measureme
nts together with local computers for data
acquisition and then use the new data immediately for controlling all loops of the process.

•Alternate quickly among standard control strategies and readjust controller parameters in
software.

• A sight full eng
ineer can use the flexibility of the framework to implement his latest
controller design ideas on the host computer or on the main control computer.


In the common DCS architecture, the microcomputer attached to the process are known
as front
-
end computer
s and are usually less sophisticated equipment employed for low
level functions. Typically such equipment would acquire process data from the measuring
devices and convert them to standard engineering units. The results at this level are
passed upward to t
he larger computers that are responsible for more complex operations.
These upper
-
level computers can be programmed to perform more advanced calculations.


5. Description of the DCS elements


The typical DCS system shown in Figure 3 can consists of one o
r more of the following
elements:

• Local Control Unit (LCU). This is denoted as local computer in Figure 3. This unit can
handle 8 to 16 individual PID loops, with 16 to 32 analog input lines, 8 to 16 analog
output signals and some a limited number of di
gital inputs and outputs.

• Data Acquisition Unit. This unit may contain 2 to 16 times as many analog input/output
channels as the LCU. Digital (discrete) and analog I/O can be handled. Typically, no
control functions are available.

• Batch Sequencing Un
it. Typically, this unit contains a number of external events,
timing counters, arbitrary function generators, and internal logic.

• Local Display. This device usually provides analog display stations, analog trend
recorder, and sometime video display for

readout.

• Bulk Memory Unit. This unit is used to store and recall process data. Usually mass
storage disks or magnetic tape are used.

Chemical Engineering Department King Saud University, 2002 Process Control in the
Chemical Industries 138


-

23

-

•General P
urpose Computer. This unit is programmed by a customer or third party to
perform sophisticated functions such as optimization, advance control, expert system, etc.

• Central Operator Display. This unit typically will contain one or more consoles for
opera
tor communication with the system, and multiple video color graphics display units.

• Data Highway. A serial digital data transmission link connecting all other components
in the system may consist of coaxial cable. Most commercial DCS allow for redundant

data highway to reduce the risk of data loss.

• Local area Network (LAN). Many manufacturers supply a port device to allow
connection to remote devices through a standard local area network.


6. The advantages of DCS systems

The major advantages of fun
ctional hardware distribution are flexibility in system design,
ease of expansion, reliability, and ease of maintenance. A big advantage compared to a
single
-
computer system is that the user can start out at a low level of investment. Another
obvious advan
tage of this type of distributed architecture is that complete loss of the data
highway will not cause complete loss of system capability. Often local units can continue
operation with no significant loss of function over moderate or extended periods of ti
me.

Moreover, the DCS network allows different modes of control implementation such as
manual/auto/supervisory/computer operation for each local control loop. In the manual
mode, the operator manipulates the final control element directly. In the auto mod
e, the
final control element is manipulated automatically through a low
-
level controller usually
a PID. The set point for this control loop is entered by the operator. In the supervisory
mode, an advanced digital controller is placed on the top of the low
-
level controller
(Figure 1). The advanced controller sets the set point for the low
-
level controller. The set
point for the advanced controller can be set either by the operator or a steady state
optimization. In the computer mode, the control system opera
tes in the direct digital
mode shown in Figure 1.

One of the main goals of using DCS system is allowing the implementation of digital
control algorithms. The benefit of digital control application can include:


• Digital systems are more precise.

• Digi
tal systems are more flexible. This means that control algorithms can be changed
and control configuration can be modified without having rewiring the system.

• Digital system cost less to install and maintain.

• Digital data in electronic files are easi
er to deal with. Operating results can be printed
out, displayed on color terminals, stored in highly compressed form.


7. Important consideration regarding DCS systems.


Chemical Engineering Department King Saud University, 2002 Process Control in the
Chemical Industries 139




7.1 The control loop


-

24

-

The control loop remains the same as the conventional feedback control loop, but with
the addition of some digital components. Figure 4 shows a typical single direct digital
control
-
loop. Digital computer i
s used to take care of all control calculations. Since the
computer is a digital (binary) machine and the information coming out of the process in
an analog for, they had to be digitized before entering the computer. Similarly the
commands issued by the co
mputer are in binary, they should be converted to analog
(continuous) signals before implemented on the final control element. This is the
philosophy behind installing the A/D and D/A converter on the control loop. Signal
conditioning is used to remove noi
se and smooth transmitted data. Amplifier can also be
used to scale the transmitted data if the signals gain is small. Signal generators
(transducer) are used to convert the process measurements into analog signals. The most
common analog signals used are
0
-
5 Volts and 4
-
20mA. Some of the process variables
are represented in millivolts such as those form thermocouples, strain gauges, pH meters,
etc. Multiplexers are often used to switch selectively a number of analog signals.


Figure 4: The component of a
digital control loop


All instrumentation hardware (1
-
9) is designed, selected, installed and maintained by an
instrumentation engineer. The computer is responsible for making decisions (control
actions). It can host a simple control algorithm or a more a
dvanced one. The latter can
either purchased from a commercial vendor or developed in
-
house by a process/control
engineer (See section 7.3). The terminal is the main operator interface with the control
system. The operator can use the terminal to monitor t
he control performance, adjust the
set points and tune the controller parameters.



7.2 The basic units of a digital computer

The digital computer used in DCS systems is a regular microcomputer with the simplified
components shown in Figure 5. It include
s the arithmetic unit, which carry out arithmetic
and logic commands. The control unit is the part of the computer responsible for reading
program statements from memory, interpreting them, and
Chemical Engineering Department King Saud University, 2002 Pr
ocess Control in the
Chemical Industries 140

causing the appropriate action to take place. The memory unit is used for storing data and
programs. Typical computers have Random
-
Access
-
Memory (RAM) and Read
-
Only
-
Memory (ROM). The final unit is the input/out
put interface. The I/O interface is
necessary for the computer to communicate with the external world. This interface is the
most important in the control implementation. The process information is fed to the
computer through the I/O interface and the comm
ands made by the computer are sent to
the final control element through the I/O interface.






Figure 5: A general purpose digital computer


-

25

-


In control application, the design of the I/O devices and interface is an important part of
the overall digital
control philosophy. The following subsections discuss some of these
issues.


7.2.1 Information presentation and accuracy.

The modern digital computer is a binary machine. This means that internal data and
arithmetic and logic must be represented in binar
y format. Therefore all process
information flowing into and out of the computer must also be converted to that form.
Traditionally, the computer memory location is made up of a collection of bits called a
word (register). A typical computer word consists
of 16 bits (new computers carry 32
-
bits
word). Consider, for example, the following machine number:
Chemical Engineering Department King Saud University, 2002 Process Control in the
Chemical Industries 141

16
-
bit computer word: 1011001100010100

The base

for this word is 2. Therefore, each bit has the following decimal equivalent:


Bit 1

Bit 2

Bit 3

Bit 4




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-

26

-

This means that eight specific values for the analog signal can be exactly recognize
d.
Any values interim values will be approximated according to the covered analog range
shown in the fourth column of Table 1. In this way, the error in resolution is said to be in
the order of 1/14. Assume now a 4
-
bit word is available for the same analog

signal. Then
the full range will be divided over 15 points, i.e. sixteen equally spaced values between 0
and 1 can be recognized, and the error in resolution will be in the order of 1/30. Most
current control
-
oriented ADC and DAC utilize a 10 to 12 bit re
presentation (resolution
better than 0.1%). Since most micro
-

and minicomputers utilize at least a 16
-
bit word, the
value of an analog variable can be stored in one
Chemical Engineering Department King Saud University, 2002 Process Control in the
Chemical

Industries 142

memory word. New computers are capable of using 32
-
bit word. Therefore, new
generation of ADC and DAC with higher resolution (up to 16 to 20 bit) are emerging.


Table 1: Representation of a 0 to 1 volt analog variable using a 3
-
bit word

Binary representation

Digital Equivalent

Analog equivalent

Analog range covered

0 0 0

0 0 1

0 1 0

0 1 1

1 0 0

1 0 1

1 1 0

1 1 1

0

1

2

3

4

5

6

7

0

1/7

2/7

3/7

4/7

5/7

6/7

1

0 to 1/14

1/14 to 3/14

3/14 to 5/14

5/14 to 7/14

7/
14 to 9/14

9/14 to 11/14

11/14 to 13/14

13/14 to 14/14


7.2.2 Process interface

A typical plant with large number of variables contains abundance of process information
(data). Therefore, process information can be classified under several classes (g
roups).
Then a specialized device can be used to transfer all information of a specific class into
and out of the computer. This way designing different I/O interface for each I/O device to
be connected to the computer is avoided. In fact, most process dat
a can be grouped into
four major categories as listed in Table 2.

Table 2: Categories of process information


Type

Example

1. Digital

Relay

Switch

Solenoid valve

Motor drive

2. Generalized digital

Laboratory instrument output

Alphanumerical di
splays

3. Pulse or pulse train

Turbine flow meter

Stepping motor


-

27

-

4. Analog

Thermocouple or strain gauge (millivolt)

Process instrumentation (4


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7.2.3 Timing

The control computer must be able to keep track of time (real time) in order to be able to
initiate data acquisition operations and calculate control outputs or to initiate supervisory
optimizatio
n on a desired schedule. Hence, all control computers will contain at least one
hardware timing device. The so
-
called real
-
time clock represents one technique. This
device is nothing more than a pulse generator that interrupts the computer on a periodic
ba
sis and identifies itself as interrupting device.


7.2.4 Operator interface.

The operator interface is generally a terminal upon which the operator can communicate
with the system. Such terminals usually permit displaying graphical information. Often
the
se display consoles are color terminals for better visibility and recognition of key
variables. The operator will use the keyboard portion of the terminal to perform specific
tasks. For
example, the operator can type in requests for information or displaying trends,
changing controller parameters or set points, adding new control loop, and so on.


7.3 Digital control software


-

28

-

To make the best use of a DCS system, an advance control stra
tegy or supervisory
optimization can be incorporated in the main host computer. In the past, computer control
projects are written in assembly language, an extremely tedious procedure. Nowadays
most user software is written in higher
-
level languages such a
s BASIC, FORTRAN, C
etc. In many cases, the user is able to utilize the template routines supplied by the vendor,
and is required only to duplicate these routines and interconnect them to fit his own
application purposes. Another way is to write his own co
mplete control program and
implement it.

Other software in the form of control
-
oriented programming languages is supplied by the
vendor of process control computers. A simpler approach for the user is to utilize vendor
-
supplied firmware or software to avo
id writing programs. Currently, most DCS
manufacturers develop their own advance control and optimization software,


Chemical Engineering Department King Saud University, 2002 Process Control in the
Chemical Industries 144

which can included in the packa
ge as options. Similarly, many control algorithm
developers; (DMC, ASPEN, etc) design a special interface to allow incorporating their
own control programs into most of the commercial DCS network.



8. Conclusion


Digitally
-
based control instrumentation
represents a revolutionary change in the process
control paradigm. With digital systems the control engineer has the opportunity to go
beyond the narrow limitation of standard analog control components to construct a
system that is optimum for the informat
ion processing and control requirements of large
processes or even of entire plants. This is why many industrial plants are updating their
hardware and instrumentation systems bearing in mind that the payout times for
installation and commissioning costs i
s as a low as three to four months.

References

Conidine, D., Proces/Indsutrial Instruments and Controls Handbook,

Ogunnaike, B. and Ray, W., Process Dynamics, Modeling and Control, Oxford
University Press, UK, 1994.

Seborg, D., Edgar, T., and Mellicham
p, D., Process Dynamics and Control, Wiley &
sons, New York, 1989.


Chemical Engineering Department King Saud University, 2002 Process Control in the
Chemical Industries 145 Chemical Engineering Department King Saud University, 2002


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29

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