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Distributed Control System DEVLOPED AT RELIANCE INDUSTRIES

Prepared by : -

Chirag Chavda

Haresh Patel

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Evolution of discrete control

(relays) More recently relays has been used for control and

early electrical control was based on relays.

These relays allow power to be switched on and off

without a mechanical switch.

It is common to use relays to make simple logical

control decisions.

Modern control systems still include

relays, but these are rarely used forlogic. A relay is a simple device that

uses a magnetic field to control a

switch.

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When a voltage is applied

to the input coil, the

resulting current

creates a magnetic field.

The magnetic field pulls

a metal switch (or reed) towards

it and the contacts touch,

closing the switch. The contact

that closes when

the coil is energized is called

normally open. The normally closed

contacts touch when the input coilis not energized. When the input coil

is not energized the normally closed

contacts will be closed (conducting).

Contd..

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What is Relay ?

A relay is a simple device that uses magnetic fieldto control a switch.

In the fig.1 we see a 4 - pin relay. Relays have twocircuits: A control circuit (shown in GREEN) and aload circuit (shown in RED). The control circuit hasa small control coil while the load circuit has aswitch. The coil controls the operation of theswitch.

Current flowing through the control circuit coil (pins1 and 3) creates a small magnetic field whichcauses the switch to close, pins 2 and 4 as in fig.2.

The switch, which is part of the load circuit, is usedto control an electrical circuit that may connect to it.Current now flows through pins 2 and 4 shown inRED, when the relay is energized.

Fig, 1

Fig, 2

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Contd..

ACTUAL RELAY DESIGN

Current flows through thecontrol coil, which iswrapped around an iron

core. The iron coreintensifies the magneticfield. The magnetic fieldattracts the upper contactarm and pulls it down,closing the contacts andallowing power from thepower source to go to theload.

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Contd..

INSTRUMENT

TASK 1

(RELAY 1)

TASK 2

(RELAY 2)

TASK 3

(RELAY 3)

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PLC evolution

PLC was developed keeping the limitationsof the relay based control.

In the late 1970s with the introduction ofpthe basic capabilities of PLC were greatlyexpanded and improved. They were able toperform sophisticated math and datamanipulation functions, which greatlyincreased the use of programmablecontrollers in more complex controlapplications.

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PLC Defined

The National Electrical ManufacturersAssociation (NEMA), USA, definesprogrammable controller as a digitalelectronic apparatus with a programmablememory for storing instructions toimplement specific functions, such as logic,sequencing, timing, counting andarithmetic, to control machines andprocesses.

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Advantages ofPLC

Smaller physical size than hard-wiresolutions.

Cost effective for controlling complexsystems.

Easier and faster to make changes. PLCs have integrated diagnostics and

override functions. Diagnostics are centrally available. Applications can be immediately

documented.

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Contd..

Applications can be duplicated faster and lessexpensively.

Computational abilities allow more

sophisticated control. Trouble shooting aids make programming

easier and reduce downtime. Reliable components make these likely to

operate for years before failure. Ability to communicate with computer

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Application

Sequence control, timing, counting, datacalculation

Batch or continuous process control. Open loop or feedback process control,

process data acquisition and display. Precise motion / position control.

Safety and Interlocks Alarm handling Sequence Of Events (SOE)

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Components ofPLC Systems

POWER

SUPPLY

PROCESSOR

PROGRAMMING

DEVICEMEMORY

I/O SYSTEMINPUTS

OUTPUTS

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How PLCs Work ?

All PLCs contain a Central Processing Unit (CPU), Memory, Power supply,Input / Output modules and programming device.

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PLC Operation

A PLC works by continually scanninga program. We can think of this scan

cycle as consisting of 3 important steps. There are typically more than 3 but we

can focus on the important parts and not worry about the others. Typically the

others are checking the system and updating the current internal counter and

timer values.

Step 1-CHECK INPUT STATUS-First the PLC takes a look at each input to

determine if it is on or off. In other words, is the sensor connected to the first

input on? How about the second input? How about the third... It records thisdata into its memory to be used during the next step.

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Contd..

Step 2-EXECUTE PROGRAM-Next the PLC executes your program one

instruction at a time. Maybe your program said that if the first input was on

then it should turn on the first output. Since it already knows which inputs

are on/off from the previous step it will be able to decide whether the first

output should be turned on based on the state of the first input. It will storethe execution results for use later during the next step.

Step 3-UPDATE OUTPUT STATUS-Finally the PLC updates the status of theoutputs. It updates the outputs based on which inputs were on during thefirst step and the results of executing your program during the second step.Based on the example in step 2 it would now turn on the first outputbecause the first input was on and your program said to turn on the firstoutput when this condition is true.

After the third step the PLC goes back to step one and repeats the stepscontinuously. One scan time is defined as the time it takes to execute the 3steps listed above.

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A Level Application

Let's consider the following application:

We are controlling lubricating oil being dispensed from a tank. This is possibleby using two sensors. We put one near the bottom and one near the top, as

shown in the picture.

Here, we want the fill motor to pump lubricating oil into the tank until the high

level sensor turns on. At that point we want to turn off the motor until the level

falls below the low level sensor. Then we should turn on the fill motor andrepeat the process.

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Here we have a need for 3 I/O (i.e. Inputs/Outputs). 2 are inputs (the sensors)

and 1 is an output (the fill motor). Both of our inputs will be NC (normally

closed) level sensors. When they are NOT immersed in liquid they will be ON.

When they are immersed in liquid they will be OFF.

We will give each input and output device an address. This lets the plc knowwhere they are physically connected. The addresses are shown in the following

tables:

Inputs Address Output Address Internal Utility Relay

Low 0000 Motor 0500 1000

High 0001

A Level Application

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Below is what the ladder diagram will actually look like. Notice that we

are using an internal utility relay in this example. You can use the

contacts of these relays as many times as required. Here they are used

twice to simulate a relay with 2 sets of contacts. Remember, these relays

DO NOT physically exist in the plc but rather they are bits in a register

that you can use to SIMULATE a relay.

A Level Application

http://opt/scribd/conversion/tmp/scratch2745/D:/spm/Presentations/plc/PLC%20Animation.htm

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Ladder Diagram (LAD)

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Scan time

Star output

scan

Start cycleOutput scan Input Scan

Start program

scan

Control program

scan

The input and output scans are normally very short relative to the time

required for the program scan

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Contd..

INSTRUMENT

(PLC)

TASK 1

(RELAY 1)

TASK 2

(RELAY 2)

TASK 3

(RELAY 3)

1000 RELAYS=1 PLC

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Triconex PLC

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PLCs in the Market

Triconex

Allen Bradley

Siemens Hitachi

GE Fanuc

Hima-Sella

Modicon

Mitsubishi

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DCS CENTUM XL DCS

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BENEFITS - DCS

Information is compressed into area that the eye can scan without head

movement.If economy permits, 2 to 3 CRTs are recommended.

One for overview display.

One for current interest group or graphics display.

One for alarm summary display.More concentration of centralized information.Ease of accessing display information through keyboard.Direct overview of entire process.Ability to anticipate incipient upsets.

Improved operator performanceHigher Degree of hardware standardization.Greater Controllability of Process Variables.

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WHY DISTRIBUTED

CONTROL?

MANAGEMENT Cost Reduction of Control Installation

By Reducing Number of Wires for signal transmission.

Inbuilt Algorithms in place of signal conditioning devices.

Less No. of Panels.

Panel Space Saving.

Control Room Design.

Cost of Heating, Ventilation, air conditioning.

Cost of Future Expansion Scalability of DCSMotorStarters

Servo motors

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Contd..

CENTUM XL DCS

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Access Control : Token Passing

Trans. Speed : 1 Mbps

Trans. Distance : 1 Km to 20km

Media : Coaxial/Optical Fibre

HF

EOPCEOPS

HF BUS

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STATION CAPACITY

Feedback Insts 255

Control Loops 80

Compu Expression 255

BSI 32

Switch Instruments 255

Valve o/p Monitoring 255

Sequence Tables 200

Internal Switches 2048

Contact Inputs 512

Contact Outputs 512

Timers 511

PresetCounters

Code Inputs

Code Outputs

Annunciator

message

255

Relational Exp 255

Sequence

Mess

255

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FCS Station display

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ENG

INEER

ING

STA

TION

SYSTEM GENERATION

ON STAND-ALONE

STATION

OPERATOR STATION

BUILDER

MOUSE DRIVEN

ISA SYMBOLS

CONTROL STATION

BUILDER

LOOP DIAG

SELF DOCUMENTATION

MULTI-WINDOW ENGS

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Powerful man /

machine

interface

Large application

capacity

Compact size

simplifies control

room layout

* 1 Second display update rate for operator

console display * Touch panel

* Powerful

graphic display

panel*Multi window

* Voice message output

*Super window

* 16- color display

* Basic language

* Upto 300 graphic display

* 16000 tags * 2,560 trend points * Compact * Support large screen CRT

EOPS

M

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MUL

TI-WIN

DOWE

OPS

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EOPS CAPACITY

FUNCTION MEMORYCAPACITY

NO. OF TAGS : max. 16000 MEMORY : 8 MB

NO. OF TREND POINTS: max2304

HARD DISK : 80MB

GRAPHIC PABNELS : 300 pages

BASIC USER AREA : 5 MB

NO. OF CRTs : upto 4

NO. OF PRINTERS : upto 4

NO. OF HARD COPIER : max 1

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Block diagram of DCS

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Contd

The arrow shows how the process has been done? Its a bi-directional process. It Controls the whole plant through control room. Its a heart of a DCS system. It shows that the how DCS is actually working? In Diagram All diagrams have their own number. And according to their numbers Process has been

done. So, this type DCS can actually work. the detail

description of it is shown into the Closed loop.

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EOPSPAN

EL

ORGANISATION

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Trend Panel

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SECURITY KEY

64 ONE-TOUCH FUNCTION

KEYS

DISPLAY PANELS

START BASIC

LOGGING REPORT

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Closed loop

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Contd..

For any Closed loop that are different parts as shown inupper figure.

In figure up to the junction box, that part is as the field side. While the other parts are at the control room.

In control room, the whole wires are comes in to the terminalbox, then it goes into the Barrier & redundancy.

Barrier provides 24V D.C. Supply, isolation, provide 4 to 20mA current and also adjust its current.

Then it goes into the Elco Board ,in this part 8 wires areconverted into 1 wire.

That wires are gone into Processor & Controller part. And this is a Bidirectional System. So, after applying logic it

goes into field side, into similar manner.

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Table of other DCS systems

Sr.no. Company name Models Year1 Yokogawa Centum series 19752 Honeywell TDC-2000,3000 1975

3 Taylor Mod III,Mod 300 1976

4 Hokushin 900/TX 19765 Foxboro SPECTRUM 1979

6 B.s.kent P4000(KELTRON) 1979 7 Fisher & porter DCF4000 1979

8 Beckman MV8000 1979

9 Fisher controls FISHER PROVOX 1979

10 Moore MYCRO 198011 Bailey NETWORK90 1980 12 Leed & Northup MAX-1 1980

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Any Queries????