PMAT Module -3 - Itipalana.org

PROCESS

CONTROL

The

process

of

recognizing

the

state

of

the

process

at

all

times,

analyse

the

information

according

to

the

set

rules

&

guidelinens

&

accordingly

actuate

the

control

elements

is

referred

to

as

process

control
.

Recognising
the Status

Process the
Information

Actuate the
Control elements

Rules &
Guidelines

In the control of process all these actions can be
taken manually with human involvement or in a
semiautomatic or fully automatic manner.

AUTOMATION

Automation

is

basically

the

delegation

of

human

control

functions

to

technical

equipment

aimed

towards

achieving
.

•
Higher

productivity

•
Superior

quality

of

end

product


•
Efficietn

usage

of

energy

&

raw

materials

•
Improved

safety

in

working

conditions

•
………
..
Etc
.

HISTORY

OF

PROCESS

CONTROL

&
AUTOMATION

MANUAL

CONTROL

HARD WIRED
CONTROL

ELECTRONIC
CONTROL

PLC
CONTROL

HISTORY

OF

PROCESS

CONTROL

&
AUTOMATION

MANUAL

CONTROL

In

this

all

actions

related

to

process

control

&

automation

are

taken

care

by

the

operators
.

One

of

the

major

drawback

of

this

method

is

the

likely

human

errors

&

consequently

its

effect

on

quality

of

the

final

product
.

The

manual

control

has

its

own

limitations

with

regard

to

mass

production

techniques

&

hence

this

method

cannot

provide

the

consumer

with

quality

goods

at

an

affordable

price
.

The

safety

or

efficient

use

of

raw

material

&

energy

are

all

subject

to

the

correctness

&

accuracy

of

human

action
.

HISTORY

OF

PROCESS

CONTROL

&
AUTOMATION

HARD

WIRED

LOGIC

CONTROL

This was considered to be first step towards automation.

Here the contactor & relays together with timers &
counters were used in achieving the desired level of
automation.

Although this method served the purpose for many
years, it had certain limitations as listed below :

•
Bulky & Complex Wiring

•
Involves lot of rework to implement changes in


control logic

•
The work can be started only when the task is fully


defined & this leads to longer project time.

HISTORY

OF

PROCESS

CONTROL

&
AUTOMATION

ELECTRONICS

CONTROL

With the advent of electronics, the logic gates started
replacing the relays & auxiliary contactors in the
control circuits.

The bimetalic & motorized timers were replaced by
electronic timers…..etc.

With incorporation of these changes, we got the
benefits of :

•
Reduced space requirements

•
Energy saving

•
Less maintenance & hence greater
reliability

•
………….etc.

HISTORY

OF

PROCESS

CONTROL

&
AUTOMATION

ELECTRONICS

CONTROL

However

even

with

electronics,

the

implementation

of

changes

in

the

control

logic

as

well

as

reducing

the

project

lead

time

was

not

possible
.

This

method

of

control

&

automation

was

also

popular

for

quite

some

time
.

HISTORY

OF

PROCESS

CONTROL

&
AUTOMATION

PROGRAMMABLE LOGIC CONTROLLER

As

the

desired

Logic

control

is

achieved

through

a

‘program’,

these

controllers

are

referred

to

as

Programmable

Logic

Controllers
.

Instead

of

achieving

the

desired

control

or

automation

through

physical

wiring

of

control

devices,

in

PLC

it

is

achieved

through

a

program

or

say

software
.

With

the

coming

of

microprocessor

&

associated

peripheral

chips,

the

whole

process

of

control

&

automation

underwent

a

radical

change
.

HISTORY

OF

PROCESS

CONTROL

&
AUTOMATION

PROGRAMMABLE LOGIC CONTROLLER

The

Programmable

Controllers

have

in

recent

years

experienced

an

unprecedented

growth

as

universal

element

in

industrial

automation
.

It

can

be

effectively

used

in

applications

ranging

from

simple

control

like

replacing

small

number

of

relays

to

complex

automation

problems
.

ADVANTAGES OF PLC

REDUCED SPACE

PLC

are

fully

solid

state

&

hence

extremely

compact

as

compared

to

hard

–

wired

controller

where

in

electro

–

mechanical

devices

are

used
.

Average

power

consumption

is

just

1
/
10
th

of

power

consumed

by

an

equivalent

relay

logic

control
.

ENERGY SAVING

EASE OF MAINTENANCE

#

Modular

replacement
.

#

Easy

Trouble

Shooting
.

#

Error

diagnostics

with

programming

unit
.

ADVANTAGES OF PLC

# Considering one time investment ; PLC is most
economical system.

ECONOMICAL

# Cost of PLC recovers within a short period (Low pay
back period).

GREATER

LIFE

&

RELIABILITY

# Static devices hence lesser number of moving parts,
reduces wear & tear.

ADVANTAGES OF PLC

GREATER

LIFE

&

RELIABILITY

#

In

the

case

of

hard

wired

logic,

the

control

hardware

is

either

electromechanical

or

pneumatic

&

therefore

it

is

more

prone

to

faults

due

to

wear

&

tear

of

moving

parts

resulting

in

lesser

ON

TIME

of

the

system
.

ADVANTAGES OF PLC

# To implement changes in control logic no rewiring is
required so considerable time is saved.

TREMENDOUS

FLEXIBILITY

# PLC can carry out complex function such as
generation of time delays, counting, comparing ,
arithmetic operations etc.

# “ On line ” (I.e. without disturbing the process ) as
well as “ Off line ” programming is possible.

#

High

processing

speed

&

great

flexibility

in

the

processing

of

both

analog

&

digital

signals
.

#

Suitability

for

closed

loop

tasks

with

several

loops

&

high

sampling

frequencies
.

APPLICATION OF PLC SYSTEMS

# In Industry , there are many production tasks which
are of highly repetitive nature. Although repetitive &
monotonous, each stage needs careful attention of
operator to ensure good quality of final product.

#

Many

times,

a

close

supervision

of

processes

cause

high

fatigue

on

operator

resulting

in

loss

of

track

of

process

control
.

#

Under

all

such

conditions

we

can

use

PLCs

effectively

in

totally

eliminating

the

possibilities

of

human

error
.


BUILDING BLOCKS OF PLC SYSTEM

The

PLC

mainly

consists

of

a

CPU,

memory

areas,

and

appropriate

circuits

to

receive

input/output

data
.

We can actually consider the PLC to be a box full
of hundreds or thousands of separate relays,
counters, timers and data storage locations.


BUILDING BLOCKS OF PLC SYSTEM

Do

these

counters,

timers,

etc
.

really

exist?


No,



They

don't

"physically"

exist

but

rather

they

are

simulated

and

can

be

considered

software

counters,

timers,

etc
.


These

internal

relays

are

simulated

through

bit

locations

in

registers
.

(more

on

that

later)

BUILDING BLOCKS OF PLC SYSTEM

BUILDING BLOCKS OF PLC SYSTEM

What does each part do?


These

are

connected

to

the

outside

world
.

They

physically

exist

and

receive

signals

from

switches,

sensors,

etc
.

Typically

they

are

not

relays

but

rather

they

are

transistors
.

INPUT RELAYS


BUILDING BLOCKS OF PLC SYSTEM

What does each part do?


INTERNAL UTILITY RELAYS


These

do

not

receive

signals

from

the

outside

world

nor

do

they

physically

exist
.

They

are

simulated

relays

and

are

what

enables

a

PLC

to

eliminate

external

relays
.


There

are

also

some

special

relays

that

are

dedicated

to

performing

only

one

task
.

Some

are

always

on

while

some

are

always

off
.

Some

are

on

only

once

during

power
-
on

and

are

typically

used

for

initializing

data

that

was

stored
.


BUILDING BLOCKS OF PLC SYSTEM

What does each part do?


COUNTERS


These

again

do

not

physically

exist
.

They

are

simulated

counters

and

they

can

be

programmed

to

count

pulses
.

Typically

these

counters

can

count

up,

down

or

both

up

and

down
.


Since

they

are

simulated

they

are

limited

in

their

counting

speed
.

Some

manufacturers

also

include

high
-
speed

counters

that

are

hardware

based
.

We

can

think

of

these

as

physically

existing
.

Most

times

these

counters

can

count

up,

down

or

up

and

down
.


BUILDING BLOCKS OF PLC SYSTEM

What does each part do?


TIMERS


These

also

do

not

physically

exist
.

They

come

in

many

varieties

and

increments
.

The

most

common

type

is

an

on
-
delay

type
.

Others

include

off
-
delay

and

both

retentive

and

non
-
retentive

types
.

Increments

vary

from

1
ms

through

1
s
.


BUILDING BLOCKS OF PLC SYSTEM

What does each part do?


OUTPUT REALYS


These

are

connected

to

the

outside

world
.

They

physically

exist

and

send

on/off

signals

to

solenoids,

lights,

etc
.

They

can

be

transistors,

relays,

or

triacs

depending

upon

the

model

chosen
.


DATA STORAGE

Typically

there

are

registers

assigned

to

simply

store

data
.

They

are

usually

used

as

temporary

storage

for

math

or

data

manipulation
.


BUILDING BLOCKS OF PLC SYSTEM

What does each part do?


DATA STORAGE

They

can

also

typically

be

used

to

store

data

when

power

is

removed

from

the

PLC
.

Upon

power
-
up

they

will

still

have

the

same

contents

as

before

power

was

removed
.

Very

convenient

and

necessary!!


RELAYS

A typical industrial relay


Now

that

we

understand

how

the

PLC

processes

inputs,

outputs,

and

the

actual

program

we

are

almost

ready

to

start

writing

a

program
.

But

first

lets

see

how

a

relay

actually

works
.

After

all,

the

main

purpose

of

a

plc

is

to

replace

"real
-
world"

relays
.


RELAYS

We

can

think

of

a

relay

as

an

electromagnetic

switch
.


Apply

a

voltage

to

the

coil

and

a

magnetic

field

is

generated
.


This

magnetic

field

sucks

the

contacts

of

the

relay

in,

causing

them

to

make

a

connection
.


These

contacts

can

be

considered

to

be

a

switch
.


They

allow

current

to

flow

between

2

points

thereby

closing

the

circuit


RELAYS

Let's

consider

the

following

example
.


Here

we

simply

turn

on

a

bell

(Lunch

time!)

whenever

a

switch

is

closed
.


We

have

3

real
-
world

parts
.


A

switch,

a

relay

and

a

bell
.


Whenever

the

switch

closes

we

apply

a

current

to

a

bell

causing

it

to

sound
.


RELAYS

Notice in the
picture that we
have 2 separate
circuits.

The bottom
(blue)
indicates
the DC part.
The top
(red)
indicates the AC
part.

RELAYS


Here

we

are

using

a

dc

relay

to

control

an

AC

circuit
.



That's

the

fun

of

relays!

When

the

switch

is

open

no

current

can

flow

through

the

coil

of

the

relay
.



As

soon

as

the

switch

is

closed,

however,

current

runs

through

the

coil

causing

a

magnetic

field

to

build

up
.
This

magnetic

field

causes

the

contacts

of

the

relay

to

close
.



Now

AC

current

flows

through

the

bell

and

we

hear

it
.

Lunch

time!