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Process Control

Process control is extensively used in industry

and enables mass production of continuous

processes such as oil refining, paper

manufacturing, chemicals, power plants and

many other industries. Process control enables

automation, with which a small staff of

operating personnel can operate a complexprocess from a central control room.

http://en.wikipedia.org/wiki/Automationhttp://en.wikipedia.org/wiki/Automation

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For example, heating up thetemperature in a room is a process

that has the specific, desired outcometo reach and maintain a definedtemperature (e.g. 20C), kept constantover time. Here, the temperature isthe controlled variable. At the same

time, it is the input variable since it ismeasured by a thermometer and usedto decide whether to heat or not toheat. The desired temperature (20C)

is the setpoint. The state of the heater(e.g. the setting of the valve allowinghot water to flow through it) is calledthe manipulated variable since it issubject to control actions.

Process Control

http://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Temperature

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A commonly used control devicecalled a programmable logiccontroller, or a PLC, is used to read a

set of digital and analog inputs, applya set of logic statements, andgenerate a set of analog and digitaloutputs. Using the example in theprevious paragraph, the roomtemperature would be an input to

the PLC. The logical statements wouldcompare the setpoint to the inputtemperature and determine whethermore or less heating was necessaryto keep the temperature constant. APLC output would then either open

or close the hot water valve, anincremental amount, depending onwhether more or less hot water wasneeded. Larger more complexsystems can be controlled by aDistributed Control System (DCS) or

SCADA system.

Process Control

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Types of control systemsIn practice, process control systems can be characterized as one or more of thefollowing forms:

Discrete Found in many manufacturing, motion and packaging applications.

Robotic assembly, such as that found in automotive production, can becharacterized as discrete process control. Most discrete manufacturing involves theproduction of discrete pieces of product, such as metal stamping.

Batch Some applications require that specific quantities of raw materials becombined in specific ways for particular durations to produce an intermediate orend result. One example is the production of adhesives and glues, which normallyrequire the mixing of raw materials in a heated vessel for a period of time to form a

quantity of end product. Other important examples are the production of food,beverages and medicine. Batch processes are generally used to produce a relativelylow to intermediate quantity of product per year (a few pounds to millions ofpounds).

Continuous Often, a physical system is represented through variables that aresmooth and uninterrupted in time. The control of the water temperature in a

heating jacket, for example, is an example of continuous process control. Someimportant continuous processes are the production of fuels, chemicals and plastics.Continuous processes in manufacturing are used to produce very large quantitiesof product per year (millions to billions of pounds).

Applications having elements of discrete, batch and continuous process control areoften called hybridapplications.

http://en.wikipedia.org/wiki/Process_control_systemhttp://en.wikipedia.org/wiki/Process_control_system

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The term SCADA usually refers to centralized systems which monitor and control entire

sites, or complexes of systems spread out over large areas ( anything from an industrial

plant to a nation). Most control actions are performed automatically by RTUs or by

PLCs. Host control functions are usually restricted to basic overriding or supervisorylevel intervention. For example, a PLC may control the flow of cooling water through

part of an industrial process, but the SCADA system may allow operators to change the

set points for the flow, and enable alarm conditions, such as loss of flow and high

temperature, to be displayed and recorded. The feedback control loop passes through

the RTU or PLC, while the SCADA system monitors the overall performance of theloop.

SCADA system

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SCADA systemA SCADA system usually consists of the following subsystems:

A humanmachine interface or HMI is the apparatus or device

which presents process data to a human operator, and throughthis, the human operator monitors and controls the process.

SCADA is used as a safety tool as in lock-out tag-out

A supervisory (computer) system, gathering (acquiring) data onthe process and sending commands (control) to the process.

Remote terminal units (RTUs) connecting to sensors in the process,converting sensor signals to digital data and sending digital data tothe supervisory system.

Programmable logic controller (PLCs) used as field devices becausethey are more economical, versatile, flexible, and configurablethan special-purpose RTUs.

Communication infrastructure connecting the supervisory systemto the remote terminal units.

Various process and analytical instrumentation

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Data acquisition begins at the RTU or PLC level and includes meter

readings and equipment status reports that are communicated to SCADA

as required. Data is then compiled and formatted in such a way that acontrol room operator using the HMI can make supervisory decisions to

adjust or override normal RTU (PLC) controls.

SCADA system

http://en.wikipedia.org/wiki/Data_acquisitionhttp://en.wikipedia.org/wiki/Data_acquisitionhttp://en.wikipedia.org/wiki/Data_acquisitionhttp://en.wikipedia.org/wiki/Data_acquisition

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Data may also be fed to an Historian, often built on acommodity Database Management System, to allow trending andother analytical auditing. SCADA systems typically implement a distributed

database, commonly referred to as a tag database, whichcontains data elements called tags orpoints.

A point represents a single input or output valuemonitored or controlled by the system. Points can beeither "hard" or "soft".

A hard point represents an actual input or output withinthe system, while a soft point results from logic andmath operations applied to other points. (Mostimplementations conceptually remove the distinction bymaking every property a "soft" point expression, whichmay, in the simplest case, equal a single hard point.)

Points are normally stored as value-timestamp pairs: avalue, and the timestamp when it was recorded or

calculated. A series of value-timestamp pairs gives thehistory of that point. It is also common to store additional metadata with

tags, such as the path to a field device or PLCregister, design time comments, and alarm information.

SCADA system

http://en.wikipedia.org/wiki/Operational_historianhttp://en.wikipedia.org/wiki/Database_Management_Systemhttp://en.wikipedia.org/wiki/Timestamphttp://en.wikipedia.org/wiki/Timestamphttp://en.wikipedia.org/wiki/Database_Management_Systemhttp://en.wikipedia.org/wiki/Database_Management_Systemhttp://en.wikipedia.org/wiki/Database_Management_Systemhttp://en.wikipedia.org/wiki/Database_Management_Systemhttp://en.wikipedia.org/wiki/Database_Management_Systemhttp://en.wikipedia.org/wiki/Operational_historian

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SCADA systems are significantly important systems used in nationalinfrastructures such as electric grids, water supplies and pipelines.However, SCADA systems have many security vulnerabilities

SCADA system

http://en.wikipedia.org/wiki/Electric_gridhttp://en.wikipedia.org/wiki/Water_supplieshttp://en.wikipedia.org/wiki/Pipeline_transporthttp://en.wikipedia.org/wiki/Pipeline_transporthttp://en.wikipedia.org/wiki/Water_supplieshttp://en.wikipedia.org/wiki/Water_supplieshttp://en.wikipedia.org/wiki/Water_supplieshttp://en.wikipedia.org/wiki/Electric_gridhttp://en.wikipedia.org/wiki/Electric_gridhttp://en.wikipedia.org/wiki/Electric_grid

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Distributed Control System (DCS)A distributed control system (DCS) refers to a control system usually of amanufacturing systemprocess or any kind ofdynamic system, in which the controllerelements are not central in location (like the brain) but are distributed throughout the

system with each component sub-system controlled by one or more controllers. DCS (Distributed Control System) is a computerized control system used to control

the production line in the industry

The entire system of controllers is connected by networks for communication andmonitoring.

DCS is a very broad term used in a variety of industries, to monitor and control

distributed equipment. Electrical power grids and electrical generation plants

Environmental control systems

Traffic signals

Radio signals

Water management systems Oil refining plants

Metallurgical process plants

Chemical plants

Pharmaceutical manufacturing

Sensor networks

Dry cargo and bulk oil carrier ships

http://en.wikipedia.org/wiki/Industrial_processhttp://en.wikipedia.org/wiki/Dynamic_systemhttp://en.wikipedia.org/wiki/Dynamic_systemhttp://en.wikipedia.org/wiki/Control_systemhttp://en.wikipedia.org/wiki/Controller_(control_theory)http://en.wikipedia.org/wiki/Industrial_processhttp://en.wikipedia.org/wiki/Dynamic_systemhttp://en.wikipedia.org/wiki/Controller_(control_theory)http://en.wikipedia.org/wiki/Electrical_power_gridhttp://en.wikipedia.org/wiki/Metallurgicalhttp://en.wikipedia.org/wiki/Chemical_planthttp://en.wikipedia.org/wiki/Sensor_networkshttp://en.wikipedia.org/wiki/Sensor_networkshttp://en.wikipedia.org/wiki/Sensor_networkshttp://en.wikipedia.org/wiki/Sensor_networkshttp://en.wikipedia.org/wiki/Chemical_planthttp://en.wikipedia.org/wiki/Chemical_planthttp://en.wikipedia.org/wiki/Chemical_planthttp://en.wikipedia.org/wiki/Metallurgicalhttp://en.wikipedia.org/wiki/Electrical_power_gridhttp://en.wikipedia.org/wiki/Electrical_power_gridhttp://en.wikipedia.org/wiki/Electrical_power_gridhttp://en.wikipedia.org/wiki/Electrical_power_gridhttp://en.wikipedia.org/wiki/Electrical_power_gridhttp://en.wikipedia.org/wiki/Controller_(control_theory)http://en.wikipedia.org/wiki/Dynamic_systemhttp://en.wikipedia.org/wiki/Dynamic_systemhttp://en.wikipedia.org/wiki/Dynamic_systemhttp://en.wikipedia.org/wiki/Industrial_processhttp://en.wikipedia.org/wiki/Control_systemhttp://en.wikipedia.org/wiki/Control_systemhttp://en.wikipedia.org/wiki/Control_system

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PLC & DCSc In the past the strength and weakness of both the automation

technologies were well understood. So it was easy forengineers to select the one that suits their application, butnow due to the advent of microprocessors both thetechnologies are merging.The requirements which arethought to be exclusive to one technology can be found in theother one too.But still there are some things which are to bechecked out before selecting the one that suits yourapplication the best.

Now a days you cannot really tell the difference between aPLC or a DCS. Since the PLC was integrated with Analog I/O it

crosses the boundary of being just digital and crosses to therealm of DCS in handling Analogs, Bus Systems, DistributedI/O and etc. Also, since the DCS now handles logics of DigitalI/O it also crossed the boundary to the realm of PLC.

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As you know PLC as to its name Programmable Logic Controller. Its mainpurpose is to replace the relay logic controls which is "On" or "Off". And DCS"Distributed Control Systems" its emphasis is Fast analog handling because of

communications through Bus systems, networking and etc.Summarizing all these, PLC = DCS......BUT still there are basic differences as

below

PLC is a programmable Logic controller which is used mainly for interlockingdifferent equipments.

PLC using for perticular mechine or production unit. PLC is economicaly low cost.

PLC'S can handle analog and digital I/O as earliear it could handle onlydigital.

PLC'S are automatic controllers which is a substitute to hard wired

controllers. they are extensively used for automation.

PLC is for stand alone sytem.

PLC is used Commonly with On\Off (Digital) Control and may be expandedwith Analog I\O Modules for Analog Control and used for a control task.

PLC & DCS

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PLC

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DCS: distributed control systems: this system does what aPLC would do, but the difference is that a DCS is used in

much larger and complex application e.g: Power Generation

Some Cold Rolling Mills etc.

DCS is the System in which controller are distributedgeographically and integrated all the control hardwares.which is connected from the various field devices .DCShaving its own network, Controller and HMI etc.

Honeywell, Yokogawa, Invensys, ABB, Emerson are theleading .DCS Vendors.DCS is the System in whichcontroller are distributed geographically and integrated allthe control hardwares which is connected from thevarious field devices.DCS having its own network,

Controller and HMI etc

PLC & DCS

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DCS

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If taken from a narrow point of view, maybe PLC and DCS are functionally thesame. Of course, for huge plants with I/O's ranging from 10,000 to 30,000 I/Opoints, and beyond, it is silly to even think about PLC.

1. A PLC is cost-effective up to a certain I/O count, and so is the DCS. But thedifference is in their starting points: the PLC is cost-effective from 0 to a fewthousand I/O points; the DCS becomes cost-effective starting from a fewthousand points and beyond.

2. A PLC becomes a subsystem of the DCS in rare occasions when the situation

calls for it, i.e., purchase of huge package systems with engineering schedulesincompatible with the DCS schedule (I/O lists cannot be submitted on timebefore the DCS hardware freeze date). Note that this package system is aprocess system using continuous control, not discrete. Based on this, a PLC cannever be larger than a DCS in terms of I/O count.

3. In large plants the DCS is king because most owners want a single source ofhardware support and service, and this mentality naturally denies the PLC afoothold. Package vendors are no longer required to provide PLC for theirsystem. Everything is connected to the DCS.

PLC & DCS

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SCADA

SCADA: AS ALREADY EXPLAINED, IT NOTHING BUTA SOFTWARE USEDTO GATHER DATA FROM THE FIELDSINSTRUMENTS WITH THE HELP OF

YOUR DCS OR PLC DEPENDING ON THEAPPLICATION VIA THE SERVERAND MAKING IT AVAILABLE IN AN ANIMATEDFORM. THIS IS

USUALLY FOUND IN YOUR CONTROL ROOMS ANDDEVELOPMENT ROOM

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SCADA

SCADA is the Software tool in which you can able to view/Monitor/Control your Process

Variable data and Graphical representation of your plant etc.

It's an operator front end display Example of few SCADA S/W is Wonderware, IFix, RS View,WinCC etc .

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PLC:

in manufacturing processes(for production of things)

for simple batch control

intensive logic controls

can handle a no. of I/O s at a time

downtime => lost production

usually, no need of redundancy

fast logic scan(=reading I/Ps executing instructions andproviding O/Ps)

compact

easily customised

for smaller applications

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DCS

in process control (for production of stuff)

for complex batch control

advanced analog loop controls

downtime=>lost production+damage to processequipment+ may lead to dangerous conditions

so Redundancy must

large in size

cant be easily customised for larger applications and plant-wide

In detail

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