PLC : Introduction

A programmable logic controller, which is the usually called a PLC or programmable

controllers is solid- state, digital, industrial computer. Upon first glance, a programmable

controller may seen to be no more than black box with wires bringing signals in and other wires

sending signals out. It might also appear there some magic being done inside that somehow

decides when field devices should be turned on. In actuality, there is no magic. PLC is a

computer and someone had to tell it what to do. The PLC knows what to do through a program

that was developed and then entered into its memory. The PLC is a computer, however

without a set of instructions telling it what to do, it nothing more than a box full of

electronic components. Without instructions, the black box that we call a PLC can do nothing.

The user program is the list of instructions that tells the PLC what to do.

1.1 What is a PLC?

The PLC can be classified as a solid state member of the computer family. A programmable

controller is an industrial computer in which control devices such a limit switches, push buttons

proximity or photoelectric sensors. Float switches or pressure switches to name a few, provide

incoming control signals into the unit. An incoming control signals is called an input.

Inputs interact with instructions specified in the under ladder program which tells the PLC how

to react to the incoming signals. The user program also directs the PLC on how to control field

device like motor starters, pilot lights, and solenoids. A signal going out of the PLC to control a

field device is called an output. Figure 1 gives an overview of the interaction between the system

inputs, PLC and its ladder program, and the pilot light output.

A formal definition of a PLC comes from the National Electrical Manufacturers Association

( NMEA) : A programmable controller is a digitally operated electronic system designed

for use in an industrial environment, which uses a programmable memory for the internal

storage of use-orientation instructions for implementing specific functions such as logic,

sequencing, timing, counting, and arithmetic to control, though digital or analog inputs and

outputs, various types of machines or processor.

1.2 Features

The PLC is the hardened industrial computer. A PLC is made so that it can survive in the

manufacturing environment. Notebook and desktop personal computers are not designed for

continuous use in the manufacturing environment. When a computer needs to reside in the harsh

manufacturing environment, an industrially hardened computer that is designed to withstand

stress is the correct choice. Industrial computers are designed to withstand the dirt, shock,

vibration, high temperatures and wash downs found in the factory environment by incorporating

the following features:-

1. PLC is small, easy to install units. A PLC is easy to install because all inputs and outputs

connections are connected to terminals strips in a central location.

2. Air filters on intake fans.

3. Fans with ball bearing.

4. National electrical manufacturers Association (NMEA) 12,4 and 4X enclosure rating.

5. Shock mounted hard drives.

6. 0 to 55 degree centigrade compared to 0 to 40 degree centigrade operating environment.

7. Industrial computers are modular, which results in faster and easier repair.

8. Industrial computers do not need to be placed in a fan- cooled or air- conditioned

enclosure with a viewing window.

9. Easier troubleshooting.

10. Total Response time = Input response + Programme execution time + output response

time.

1.3 History of PLC

Early machines were controlled by mechanical means using cams gears, levers and other basic

mechanical devices. As the complexity grew, so did the need for a more sophisticated control

system. These elements were wired as required to provide the control logic necessary for the

particular type of machine operation. Relay and switch logic was cumbersome and time

consuming to modify. Wiring had to be removed and replaced to provide for the new control

scheme.

This modification was difficult and time consuming to design and install and any small “bug” in

the design could be a major problem to correct since it also required rewiring of the system. A

new means to modify control circuitry was needed. The Development and testing ground for this

new means was U.S auto Industry. The time period was the late 1960s and early 1970s and the

result was the PLC. Automotive plants were confronted with a change of manufacturing

techniques every time a model changed and, in some cases, for changes on the same model if

improvements had to be made during the model year. The PLC provided an easy way to

reprogram the wiring rather than actually rewiring the control system.

The PLC that was developed during this time was not very easy to program. The language was

cumbersome to write, requiring highly trained programmers. These early devices were merely

relay replacements and could do very little else. Older PLCs were capable of only handling

discrete inputs and outputs (i.e. ON-OFF type signals). While today’s systems can accept and

generate analog voltages and currents as well as wide range of voltage levels and pulsed

signals. Unlike their personal computer cousin, they can typically withstand the vibration, shock,

elevated temperature, and electrical noise to which manufacturing equipment is exposed.

As more manufactures become involved in PLC production and developments, and PLC

capabilities expand, the programming language is also expanding. Also, manufacturers tend to

develop their own versions of ladder logic language.

Comparison

PLC MICROPROCESSOR MICROCONTROLLLER

1. A plc is a type of 1. A microprocessor is the 1. A microcontroller is a special

computer designed

specifically for industrial

applications.

Central Processing Unit

(CPU) of a computer.

purpose computer system, usually

programmed to perform a single

task.

1. All PLCs contain

microprocessors.

2. Not all microprocessors

are used in PLCs.

2. Microcontrollers have

microprocessors as a part of their

system hardware.

2. A PLC can be stated as a

complete computer in

itself with a

microprocessor.

3. A microprocessor is only

one component of an

electronic device and

requires additional

circuits, memories etc.

before it can function.

3.A microcontroller is a small task-

specific computer. They contain

microprocessors on-board to

handle logic and instruction

processing but they still must have

all the essential elements of any

computer system.

4. A PLC can be

programmed or re-

programmed to control

different types of

devices using Ladder Logic

et c.

5. A general computer or

microprocessor system

can be tasked with a wide

variety of jobs.

4. Usually a microcontroller is

a programmed for a specific

task and left alone to do it

without further human input.

6. Applications include

automation and process

based industries etc.

7. Applications include

computers, telephone

industry home appliances

such as microwave.

5.Applications include

monitoring water level in a

tank etc.

Hardware Components:

There are two fundamental uses of switches. First, switches are used for operator to send

instructions to the control circuit. Second, switches may be installed on the moving parts of a

machine to provide automatic feedback to the control system.

Push Button: The most common switch is the push button.

It is also the one that needs the least description because it is widely used in an automotive.

The momentary push button switch is activated when the button is pressed and deactivated

when the button is released. The deactivation is done using an internal spring.

The maintained push buttons switch activates when press, but remains activated when it is

released. To deactivate it, it must be pressed a second time. For this reason, this type of switch is

sometimes called a push- push switch.

The contacts on switches can be of two types: There are normally open (N/O) and normally

closed (N/C). Whenever, a switch is in its deactivated position, the N/O contacts will be

open ( non- conducting ) and the N/C contacts will be closed ( conducting ) shows the

schematic symbols for (a) a normally open push buttons and (b) a normally closed ( push-

Buttons. There is no internal electrical connection between different contact pairs on the same

switch. Most industrial switches can have extra contacts “piggy backed” on the switch, so as

many contacts as needed of either type can be added by the designer.

Selector Switches: A selector switch is also known as a rotary switch. An automobile ignition

switch and an oscilloscope’s vertical gain and horizontal time base switches are examples of

selector switches. Selector switches use the same symbol as a momentary pushbutton, except

a lever is added to the top of the actuator as shown. The switch is open when the selector is

turned to the left and closed when turned to the right. The switch has two sets of contacts. The

top contacts are closed when the switch selector is turned to the left position and open when the

selector is turned to right. The bottom set of contacts works exactly opposite. There is no

electrical connection between the top and bottom pairs of contacts. For the switch, the control

panel would be labeled with the STOP position to the left and the RUN position to the right.

Metal Detector Switch: It is another hardware component that works same as that of push-button

being sensitive to metal. It gets turned on when it gets into contact with metal and gets turned

off when the metal contact is removed.

MANUFACTURE AND CLASSIFICATION

Manufacturer PLC Memory I/D Count Communications

options

General Electric VersaMax Nano 2 K 6 input

4 Output

Serial

Siemens Simatic S7- 200,

CPU 221

4 K Program 2 K

data

6 input

4 Output

Optional RS 485

Mitsubishi FXO 1.6K Up to 16

inputs

Up to 14

outputs

Serial, Profibus-

DP,CC- Link

Network

Rockwell MicroLogix 1 K Up to 20

inputs

Up to 12

outputs

Serial, Data

Highway 485,

DeviceNet

General Electric VersaMax

Micro

9 K Up to 84 Serial, RS-485

Omran CPMNM1A 3 K 100 Host link, NT

Link, 1:1 Link

Mitsubishi FX2N Super

Micro

Up to 16 K Up to 256 Serial and RS-

422/485 Profibus-

DP and CC- Link.

Omran CQMI 13 k Up to 256 Device, Net.

CompoBus/ S;

Host Link, NT

Link,, 1:1 Link

Siemens Simatic S7-300 6 to 512 K Upto 512

discrete or

128

analog

Point-to – point

links, AS-

Interface,

profibus- DP,

Profibus- FMS,

Industrial

Ethernet

Rockwell SLC 500 Up to 64 K Up to

4096

inputs and

outputs

Serial DH- 485,

DH+, Device Net,

Control Net,

Ethernet.

Omran Cvmi Up to 62 K Up to

2048

Ethernet, Sysmac-

net, Sysmatic-

Link controller

Link, Device,

Nete, Host Llink,

NT Link.

Mitsubishi AnN Series Up to 320 K Up to

2048

Modbusm

Profibus- DP,

Ethernet

Rockwell ControlLogix 750- Kthru 8 M

bytes

Up to

128.000

Serial DH- 485,

DH+, Device Net,

Control Net,

Ethernet

Plc Manufacturing Companies

Modicon from France

Allen Bradely USA

Siemens Germany

ABB USA

OMRON Japan

Mistubushi South Korea.

GE Fanuc South Korea

LG Japan

Phillip U.K

Yokogava Japan

3.2 Types of PLC

1. Compact type (Number of input are fined). e.g. Zelio, Twido.

2. Modular type or rack type ( Number of input increase or decrease according to

requirement).e.g. Allen bradley

3.2.1 Zelio PLC

Manufacturer : Schneider Electrical

Software : Zelio soft 2

Twido PLC

Manufacturer : Schnieder

Software : Twido soft

Comm Protocol : Modbus urial

Connector : 8 Pin Mini Din

Comm port : RS 485

Plc Series : TWDLCAA 24DRF

PROGRAMMING A PROGRAMMABLE CONTROLLER

The PLC can do nothing without someone developing a program and loading this user program

into the CPU’s memory. Once the CPU has the program in memory and has been put into run

mode, it can look at inputs and a result of solving the user program ladder logic instructions it

can control the outputs and their associated field devices.

There are multiple ways to program a PLC.

1. One of the oldest methods of programming a PLC is by pressing buttons on a handheld

programming terminal to enter a user program a PLC.

2. The most popular method of PLC programming is using a personal desktop computer

and either a DOS or Windows operating system to run the manufacturers software for the

specific PLC. Illustrates the choice of an SLC 500 handheld terminal or a personal

computer for programming. Allen Bradley SLC 500 programming options. Either a

handheld programmer or a personal computer can be used to program the PLC. (Used

with permission of Rockwell Automation).

3. PLC programming can be accomplished using a notebook personal computer running

PLC programming software and a Personal computer Memory Card International

Association ( PCMCIA) interface card, or in some cases, a direct connection between the

personal computer serial port and the PLC CPU.

4. Using the industrial computer and the PLC manufacturer’s program software.

5. Using third- party “open software” and running a personal computer as the PLCs CPU.

4.1 Programming Devices

A program device is needed to enter, modify and troubleshooting the PLC program, or to check

the condition of the processor. Once the program has been entered and the PLC is running, the

programming device may be disconnected. Three types of programmers are generally used

1. Hand held ( smaller, cheaper portable but limited display capability and few

functions)

2. Dedicated desktop user- friendly, designed or industrial use, portable but costly,

limited PLCs can be programmed, limited documentation and limited

graphics capability.

3. Personal computer with software available for all major brands of PLCs, the PC

today is the most common programming device. It can store program on floppy

disc/hand disc. If for some reason the program is lost the restoration of the

program is simple.

On line programming changing programs while the processor and driven equipments running,

must be done by persons with a complete understanding of circuit operation and the process or

driven equipment.

Off line programming the programming is being developed off line (without being connected to

the process or driven equipment). Since few program are ever created without mistakes, it is the

most common and safest method of programming. After testing and verifying the program

offline, the PLC can be put in the ON LINE mode for final verification and operation.

4.2 Programming Languages of PLC

IEC 1131-3 is the international standard for programmable controller programming languages.

The following is a list of programming languages specified by this standard:

1. Ladder diagram (LD)

2. Sequential Function Charts (SFC)

3. Function Block Diagram (FBD)

4. Structured Text (ST)

5. Instruction List (IL)

One of the primary benefits of the standard is that it allows multiple languages to be used within

the same programmable controller. This allows the program developer to select the language best

suited to each particular task.

Ladder Logic

Ladder logic is the main programming method used for PLC's. As mentioned before, ladder logic

has been developed to mimic relay logic. The decision to use the relay logic diagrams was a

strategic one. By selecting ladder logic as the main programming method, the amount of

retraining needed for engineers and trades people was greatly reduced.

The first PLC was programmed with a technique that was based on relay logic wiring

schematics. This eliminated the need to teach the electricians, technicians and engineers how to

program - so this programming method has stuck and it is the most common technique for

programming in today's PLC.

Mnemonic Instruction

There are other methods to program PLCs. One of the earliest techniques involved mnemonic

instructions. These instructions can be derived directly from the ladder logic diagrams and

entered into the PLC through a simple programming terminal.

Sequential Function Charts (SFC)

SFC have been developed to accommodate the programming of more advanced systems. These

are similar to flowcharts, but much more powerful. This method is much different from

flowcharts because it does not have to follow a single path through the flowchart.

Structured Text (ST)

Programming has been developed as a more modern programming language. It is quite similar to

languages such as BASIC and Pascal.

Structured Text (ST) is a high level textual language that is a Pascal like language. It is very

flexible and intuitive for writing control algorithms.

Function Block Diagram (FBD)

FBD is another graphical programming language. The main concept is the data flow that start

from inputs and passes in block(s) and generate the output.

4.3 Rung

1. A rung of ladder diagram code can contain both input and output instructions.

a) Input instructions perform a comparison or test and set the rung state based on the

outcome.

b) Normally left justified on the rung.

2. Output instructions examine the rung state and execute some operation or function.

a) In some cases output instructions can set the rung state.

b) Normally right justified on the rung.

Fig 4.1 Rung

4.4 Logics for Basic Gates

Table III Logics Symbols

LOGIC SYMBOLS

logic element AND OR NOT NAND NOR

Logic

Element

Function

Output If All

Of the Control

Inputs On

Output if any

one of the

control input is

on

Output if

single control

input signal

is off

Output if all

control input

signals are on

Output if any

of the control

inputs are on

MIS-STD 80

6B Logic

Symbol

Electric Relay

Logic Symbol

Electrical

Switch Logic

Symbol

4.5 Timer and Counter

One of the major enhancements to the original programmable controller was to add timing and

counting abilities. Early PLCs had optional timing circuit cards that the user could slide into the

CPU to add timer or counter functionality to the PLC. Timing cards had physical solid- state

timing chips installed on slide- in timer boards. Today’s PLC uses modern microprocessor

technology and have timers and counters included in the instructions set.

4.5.1 Timer

A timer consists of the following parts: timer address, preset value, time base and

accumulated value. There are bits associated with the current state of the timer called status

bits. The timer address is a timer unique identifier in PLC memory. A timer instruction is one

element. A timer element is made up of three 16- bits words:-

1. Word zero contains the three states bit, EN, TT and DN (status bits will be covered as we

introduce each timer instruction).

2. Word one is for the present value.

3. Word two is for the accumulated value.

4.5.1.1 The ON Delay Timer Instruction

Uses the on- delay timer instruction program a time delay before an instruction becomes true.

Timer Addressing

Timer addressing is as follows: T (Timer file number): (Timer element number). The timer

address T4:0 is addressing timer file 4, timer element 0.

4.5.1.2 OFF- Delay Timer Instruction

Uses the off-delay timer instructions if you want to program a time delay to being after rung

inputs go false. As an example, an external cooling fan on a motor is to run all the time the motor

is running and for 100 seconds after the motor is returned off. This involves a 100- second off-

delay timers. The 100- second timing cycle begins when the motor is turned off. Figure

illustrates an off- delay timer, which Allen Bradley calls a timer off delay and its associated

ladder rungs.

TIMER INSTRUCTIONS

Instructions Use this instruction to Functional description

On- Delay Program a time delay to

before instruction

become true

When you want an action to begin a specified

time after the input become true.

Off- Delay Program a time delay to

begin after rung inputs to

false.

If an external cooling fan on a motor is to run all

the time the motor is running and for five minutes

after the motor is turned off, you have a five

minute off- delay timer. The five- minute timing

cycle begins when the motor is turned off.

Retentive Retain accumulated value Use a retentive timer to track the running time of

through power losses,

processor mode change.

Or rung state going from

true to false

a motor for maintenance purposes. Each time the

motor is turned off, the timer will remember the

elapsed time. Next time the motor is turned on,

the time will increase from that point. When you

want to reset this timer, use a reset instruction.

Reset Reset the accumulated

value of a timer or

counter

Typically used to reset a retentive timer’s

accumulated value to zero.

4.5.2 Counter

Every PLC has counter instructions Although most PLC counter work the same the instructions

symbols used and method of programming will change for different manufactures. The typical

counter counts from 0 to a predetermined values, called the ‘preset” value. The counter which

counts from 0 to a desired value is called count-up or up-counter and the counter which counts

down is called a desired value to 0 is called down- counter or count- down counter. A counter

instruction is one element. A counter element is made up of three 16- bits words. Thus the

counter instructions contain three parts.

4.5.2.1 The Count-Up Instruction

Use the count-up instruction if you want to counter to increment one decimal value each time it

registers a rung transition from false to true.

4.5.2.2 The Count Down Instruction

Use this instruction if you want to count down over the range of + 32, 767 to – 32,768. Each time

the instruction sees a false-to-true transition, the accumulated value will be decremented by one

count.

COUNTER INSTRUCTIONS

Instructions Use this instruction to Functional Description

Count up Count from zero up to a

desired value

Counting the number of parts produced during a

specific work shift, or in the current batch. Also,

counting the number of rejects from this batch.

Count down Count down from a desired

value to zero

An operator interface display shows the operator the

number of parts remaining to be made for a lot of 100

parts ordered.

SCOPE OF PLC

A PLC (Programmable Logic Controller) is one of the main devices used in industry to

implement: monitoring, logic, control, or other events/functions impossible (or to complicated)

to be done mechanically. With respect to embedded systems, a PLC is in fact an embedded

system running a program to provide the various functions PLCs typically provide.

5.1 Advantages of PLC

1. Flexibility: One single Programmable Logic Controller can easily run many machines.

2. Correcting Errors: In old days, with wired relay-type panels, any program alterations

required time for rewiring of panels and devices. With PLC control any change in circuit

design or sequence is as simple as retyping the logic. Correcting errors in PLC is

extremely short and cost effective.

3. Space Efficient: Today's Programmable Logic Control memory is getting bigger and

bigger this means that we can generate more and more contacts, coils, timers, sequencers,

counters and so on. We can have thousands of contact timers and counters in a single

PLC. Imagine what it would be like to have so many things in one panel.

4. Low Cost: Prices of Programmable Logic Controlers vary from few hundreds to few thousands. This is nothing compared to the prices of the contact and coils and timers that you would pay to match the same things. Add to that the installation cost, the shipping cost and so on.

5. Testing: A Programmable Logic Control program can be tested and evaluated in a lab.

The program can be tested, validated and corrected saving very valuable time.

6. Visual observation: When running a PLC program a visual operation can be seen on the

screen. Hence troubleshooting a circuit is really quick, easy and simpler.

7. Changes and error correction system easier: If one system will be modified or

corrected, the change is only done on the programs contained in computers, in a relatively

short time, after that it downloaded to the PLC. If not using a PLC, for example relays the

amendments made by altering the wiring cables. This course takes a long time

8. Cheaper: PLC is capable of simplifying a lot of cabling compared to a relay. So the price

of a PLC at a price cheaper than some fruit relay capable of doing the wiring for the same

amount with a PLC. PLC includes relays, timers, counters, sequencers, and other

functions.

9. Operating speed: PLC operation speed is faster than the relay. Speed PLC scan time is

determined by its in units of milliseconds.

10. Resistant character test: Solid state devices are more resistant than the relay and test

mechanical or electrical timers. PLC is a solid state device that is more resistant test.

11. Simplifies the control system components: The PLC also have counters, relays and

other components, so it does not require components such as additional. Use of relays

requires counters, timers or other components as additional equipment.

12. Documentation: Printout of the PLC can be directly obtained and do not need to see the

blueprint of his circuit. Unlike the printout relay circuit cannot be obtained.

13. Security: Changing the PLC cannot be done unless the PLC is not locked and

programmed. So there is no unauthorized person can change the PLC program for a PLC

is locked.

14. Can make changes by reprogramming: Since the PLC can be programmed quickly

reset the production process that mixes can be completed. For example part B will be

executed but sections of A is still in the process, the process in section B can be re-

programmed in seconds.

15. Addition of faster circuits: Users can add a circuit controller at any time quickly,

without requiring great effort and cost as in conventional controllers.

5.2 Disadvantages of the PLC

1. The drivers of Hilo in the area of weighbridge queue have to keep their idling of engines and

their pressure up of brake in light option anticipation.

2. It is not likely for the system to distinguish between burnt and unburnt cane. It means that

the favoring old system of one tandem or the burnt cane has must be discarded

3. There's too much work required in connecting wires.

4. There's difficulty with changes or replacements.

5. It’s always difficult to find errors; and require skillful work force.

6. When a problem occurs, hold-up time is indefinite, usually long.

7. 5.3 Limitations of PLC

The system now has some limitations even though these could be reduced by program changes

or method changes the allocation of light sequence:

1. The hauliers with interconnects have a advantage of payload over standard trailers as the

light ratio options in the sequence is presently based on loads number only, and is not

weighted based on the payload differences.

2. If there is haulier with a small tonnage would occurrence huge delays if allocated a detailed

light.

3. It should a vehicle progress to the wrong system tandem is ignorant of this and another load

is not called even though it is requisite. This trouble could be programmed out with a routine

of time delay.

5.4 Trouble Shooting

Careful start up procedures is necessary to prevent the damage to the driven equipment and PLC

or more important injury to personal. Prior to binging a system start up procedure, it is important

to check and verify that the system has been installed according to the manufacturers

specifications and that the installation, meets local, state and national codes. Special attention be

given to system grounding.

Before applying power to the controller following is to be ensured

1. Verifying that incoming power matches the jumper selected voltage setting of the power

supply.

2. Verify that a hardwired safety circuit or other emergency stop device has been installed and

is in open position.

3. Check all power and communication cables to ensure that connector pins are straight and not

bent or pulled out.

4. Connect all cables making sure that connectors are fully inserted into their sockets secure

connectors as applicable.

5. Ensure that modules are securely held in the I/O rack.

SCADA: 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 (PLCs), 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.

SCADA runs on a PC and is generally connected to various PLCs and other peripheral devices.

It enables you to generate applications for the most demanding requirements of plant engineers,

operators, managers tailored precisely to the needs of each plant. SCADA constantly gathers a

data from the plant in real-time, stores and processes it in the database, evaluates and

generates alarms. Displays information to plant operators to plant operators, supervisors and

managers and can issued instructions to PLCs on the plant floor. SCADA systems used to run on

DOS, VMS and UNIX; in recent years all SCADA vendors have moved to NT and some also to

Linux.

Features of SCADA

1. Dynamic Process graphic mimics developed in SCADA software should resemble the

process mimic. SCADA should have good library of symbols so that you can develop

the mimic as per requirements. Once the operator sees the screen he should know what

is going on in the plant.

2. Real time and Historical Trend: The trends play very important role in the process

operation. If our batch fails or the plant trips, we simply go to historical trend data and do

the analysis. We can have better look of the parameters through the trends. Ex: we

commission a SCADA system for Acid Regeneration plant where the plant has to be

operated on 850- deg temperature. If the operator operates the plant at 900 deg you can

imagine how much additional LPG he is putting into the reactor. Again what will happen

to the bricks of the reactor? So the production manager’s first job will be to go through

the trends how the operators are operating the plant. Even when the plant trips there are

more than 25 probable reasons for the same but you go through the history trends, it’s

very easy to identify the problems.

3. Alarms have a very vertical role in automation. Generally we have alarm states for each

inputs/outputs like your temperature should not cross 80 deg or lever should be less than

60. So if the parameter goes in Alarm state then operator should be intimated with the

alarm. Most of the SCADA software support our types of alarms like LOLO, LO, HI and

HIHI. Dead band the value of dead band defines the range after which a high low alarm

condition returns to normal.

4. Alarms are most important of plant control applications because the operator must know

instantly when something goes wrong. It is often equally important to have a record of

alarm and whether an alarm was acknowledged. An alarm occurs when something goes

wrong. It can signal that a device or process has ceased operating within acceptable,

predefined limits or it can indicate breakdown, wear or process malfunction.

5. Recipe Management is an additional feature. Some SCADA software support it, some

do not. Most of the plants are manufacturing multi products. When you have different

product to manufacture, you just have to load the recipe of the particular product.

6. Security is one facility people generally look for. You can allocate certain facilities or

features to the operator, process people, engineering department and maintenance

department for example operators should only operate system, he should not be able

change the application. The engineers should have access to changing the application

developed.

7. Device connectivity we will find there are hundreds of automation software manufacturer

like Modicon, Siemens, Allen Bradley, Yokogawa, ABB. Everybody has their own way

to communication or we can say they have their own communication protocol. SCADA

software should have connectivity to the different h/w used in automation. It should not

happen that for Modicon I am buying one software and for Seimens another on. The

software like Aspic or Wonderware has connectivity to almost all hardware used in

automation.

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 customers and to take advantage of new

technologies.

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 when 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 attributes and

functionality. In addition, they will also support multi-team development.

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

1. Web technology, ActiveX, Java, etc.

2. OPC as a means for communicating internally between the client and server modules. It

should thus be possible to connect OPC compliant third party modules to that SCADA

product.

Architecture :

7.4.1 Hardware Architecture

One distinguishes 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 through process

controllers. Process controllers, e.g. PLCs, are connected to the data servers either directly or

via networks or field buses that are proprietary (e.g. Siemens H1), or non-proprietary (e.g.

Profibus). Data servers are connected to each other and to client stations via an Ethernet LAN.

The data servers and client stations are NT platforms but for many products the client stations

may also be W95 machines.

7.4.2 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.

Manufacturers of SCADA

Modicon (Telemecanique) Visual look

Allen Bradly : RS View

Siemens : win cc

KPIT : ASTRA

Intelution : Aspic

Wonderware : Intouch

Usefulness of SCADA

7.6.1 Production department

1. Real time production status: manufacturing status is updated in real time in direct

communication to operator and control device.

2. Production schedules: production schedules can be viewed and updated directly.

3. Production information management: production specific information is distributed to all.

7.6.2 Quality department

1. Date integrity and quality control is improved by using a common interface.

2. It is an open platform for statistical analysis.

3. Consolidation of manufacturing & Lab data.

7.6.3 Maintenance department

1. Improved troubleshooting and de- bugging: direct connection to wide variety of the

devices, displays troubleshooting reduced diagnostic/ debugging time.

2. Plant can be viewed remotely. Notification can include pagers, e-mails, and phones.

3. C0-ordinationi between maintenance and management reduces unscheduled downtime

7.6.4 Enterprise department

1. Corporate information and real time production data can be gathered and viewe3d from

anywhere within your operations.

2. User specific information ensures better informed decision

3. Data exchange with standard database and Enterprise systems provides integrated

information solutions.

7.6.5 Engineering department

Integrated Automation solutions reduce design and configuration time

1. Common configuration platform offers flexibility for constant configuration in all areas.

2. Capable of connecting to wide variety of systems. Reduces start up time and system

training with industry proved open interfaces.

7.6.6 Manufacturing department

1. Unscheduled down time is reduced due swift alarm detection and event driven

information.

2. Makes operations easier and more repeatable with its real time functionality.

3. Secured real time operation is maintained with windows.

Application of SCADA

7.7.1 Configuration

The development of the applications is typically done in two stages. 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 appropriate to the process parameters.

7.7.2 Development Tools

The following development tools are provided as standard:

1. A 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 generic 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.

2. A data base configuration tool (usually through parameter templates). It is in general

possible to export data in ASCII files so as to be edited through an ASCII editor or Excel.

3. A scripting language

4. An Application Program Interface (API) supporting C, C++, VB.

7.7.3 Bottling of Soft Drinks

The bottling of soft-drink in a plant is done by using SCADA software. The plant used for this

bottling is highly automated and hence SCADA software finds application here. The process

makes use of the conveyer belt, on which the bottles move for filling. The motion of the

conveyer belt is so programmed such that the conveyer belt stops the bottle below the nozzle for

only that much time that is required for the bottle to get filled. After that the conveyer belt is

moved further and second bottle is undergone the same process.

The other part of this plant which is programmed is the tank containing the drink. This tank is

also programmed in a way that the liquid will come out in that much amount only which is

required to fill the bottle and this tank is in synchronization with the conveyer belt such that the

liquid will come out only when the bottle is exactly below the nozzle of the tank.

As the bottles are filled, they are carried by the same conveyer belt to the placing racks.

Automation: ''Automation'' or industrial automation or numerical control is the use of control

systems such as computers to control industry industrial machinery and industrial process

processes, reducing the need for human intervention. In the scope of industrialization,

automation is a step beyond mechanization. Whereas ''mechanization'' provided human operators

with machinery to assist them with the ''physical'' requirements of work, ''automation'' greatly

reduces the need for human ''sensory'' and ''mental'' requirements as well. Processes and systems

can also be automated.

Automation plays an increasingly important role in the global economy and in daily experience.

Engineers strive to combine automated devices with mathematical and organizational tools to

create complex systems for a rapidly expanding range of applications and human activities.

Many roles for humans in industrial processes presently lie beyond the scope of automation.

Human-level pattern recognition, language recognition, and language production ability are well

beyond the capabilities of modern mechanical and computer systems. Tasks requiring subjective

assessment or synthesis of complex sensory data, such as scents and sounds, as well as high-level

tasks such as strategic planning, currently require human expertise. In imany cases, the use of

humans is more cost-effective than mechanical approaches even where automation of industrial

tasks is possible.

Specialised hardened computers, referred to as programmable logic controllers (PLCs), are

frequently used to synchronize the flow of inputs from (physical) sensors and events with the

flow of outputs to actuators and events. This leads to precisely controlled actions that permit a

tight control of almost any industrial process.

Automation Tools: Different types of automation tools exist:

1. ANN - Artificial neural network

2. DCS - Distributed Control System

3. HMI - Human Machine Interface

4. SCADA - Supervisory Control and Data Acquisition

5. PLC - Programmable Logic Controller

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