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!
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