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Introductions to temperature control
and PID controllers
Process control
system
.
Automatic
process
control
is
concerned
with
maintaining
process
variables temperatures pressures flows
compositions, and the like at
some
desired operation value. Processes are dynamic in
nature. Changes
are always occurring,
and if actions are not
taken, the important process variables-
those related to safety, product
quality, and production rates-will not
achieve design conditions.
In order
to fix ideas, let us consider a heat exchanger in
which a
process
stream
is
heated
by
condensing
steam.
The
process
is
sketched
in
Fig.1
Fig. 1 Heat
exchanger
The
purpose
of
this
unit
is
to
heat
the
process
fluid
from
some
inlet
temperature,
Ti(t),
up
to
a
certain
desired
outlet
temperature,
T(t).
As
mentioned, the heating medium is
condensing steam.
The energy
gained by the
process fluid
is equal to
the heat
released
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by
the
steam,
provided
there
are
no
heat
losses
to
surroundings,
iii
that
is, the heat exchanger
and piping are well insulated.
In
this
process
there
are
many
variables
that
can
change,
causing
the
outlet
temperature
to
deviate
from
its
desired
value.
[21
If
this
happens,
some action must be taken to correct
for this deviation. That is, the
objective is to control the outlet
process temperature to maintain its
desired value.
One
way
to
accomplish
this
objective
is
by
first
measuring
the
temperature T(t) , then
comparing
it to
its desired value, and, based on
this comparison, deciding what to do to
correct for any deviation. The
flow of
steam can be used to correct for the deviation.
This is, if the
temperature is above
its desired value, then the steam valve can be
throttled back
to
cut the
stearr
flow (energy) to the heat exchanger. If
the temperature is below its desired
value, then the steam valve could
be
opened
some
more
to
increase
the
steam
flow
(energy)
to
the
exchanger.
All
of
these
can
be
done
manually
by
the
operator,
and
since
the
procedure
is fairly
straightforward, it should present no problem.
However, since
in most process plants
there are hundreds of variables that must be
maintained
at
some
desired
value,
this
correction
procedure
would
required a tremendous
number of operators. Consequently, we would like
to
accomplish
this
control
automatically.
That
is,
we
want
to
have
instnnnents
that
control
the
variables
wJtbom
requ)ring
intervention
from
the operator. (si This
is what we mean by automatic process control.
To accomplish ~his objective a control
system must be designed and
implemented.
A
possible
control
system
and
its
basic
components
are
shown
in
Fig.2.
Fig. 2 Heat exchanger control
loop
The first thing to do is to
measure the outlet temperaVare of the
process
stream.
A
sensor
(thermocouple,
thermistors,
etc)
does
this.
This
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sensor is connected physically to a
transmitter, which takes the output
from
the
sensor
and
converts
it
to
a
signal
strong
enough
to
be
transmitter
to
a
controller.
The
controller
then
receives
the
signal,
which
is
related
to
the
temperature,
and
compares
it
with
desired
value.
Depending
on
this
comparison,
the
controller
decides
what
to
do
to
maintain
the
temperature
at its desired
value. Base on this decision, the controller then
sends
another signal to final control
element, which in turn manipulates the
steam flow.
The preceding
paragraph presents the four basic components of
all
control systems. They are
(1) sensor, also often called the
primary element.
(2) transmitter,
also called the secondary element.
(3) controller, the
(4)
final
control
system,
often
a
control
valve
but
not
always.
Other
common final control elements are
variable speed pumps, conveyors, and
electric motors.
The
importance of these components is that they
perform the three
basic operations that
must be present in every control system. These
operations are
(1)
Measurement (M) : Measuring the variable to be
controlled is
usually done by the
combination of sensor and transmitter.
(2)
Decision
(D):
Based
on
the
measurement,
the
controller
must
then
decide what to do to
maintain the variable at its desired value.
(3)
Action
(A):
As
a
result
of
the
controller's
decision, the
system
must
then
take
an
action.
This
is
usually
accomplished
by
the
final
control
element.
As mentioned,
these three operations, M, D, and A, must be
present
in every control system.
PID controllers can be stand-alone
controllers (also called single
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loop
controllers),
controllers
in
PLCs,
embedded
controllers,
or
software
in Visual Basic or
C# computer programs.
PID
controllers
are
process
controllers
with
the
following
characteristics:
Continuous process control
Analog input (also known as
Analog output (referred
to simply as
Setpoint (SP)
Proportional (P), Integral (I),
and/or Derivative (D) constants
Examples
of
process
control
are
temperature,
pressure,
flow,
and
level
control.
For
example,
controlling
the
heating
of
a
tank.
For
simple control, you have two temperature limit
sensors (one low and
one high) and then
switch the heater on when the low temperature
limit
sensor tums on and then mm the
heater off when the temperature rises to
the high temperature limit sensor. This
is similar to most home air
conditioning & heating thermostats.
In contrast, the PID controller
would receive input as the actual
temperature and control a valve that
regulates the flow of gas to the
heater. The PID controller
automatically finds the correct (constant)
flow
of
gas
to
the
heater
that
keeps
the
temperature
steady
at
the
setpoint.
Instead of the temperature bouncing
back and forth between two points,
the
temperature is held
steady. If the
setpoint is lowered, then the PID
controller
automatically
reduces
the
amount
of
gas
flowing
to
the
heater.
If
the
setpoint
is
raised,
then
the
PID
controller
automatically
increases
the
amount
of
gas
flowing
to
the
heater.
Likewise
the
PID
controller
would
automatically
for
hot,
sunny
days
(when
it
is
hotter
outside
the
heater)
and
for cold, cloudy days.
The analog
input (measurement) is called the
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parameter
you
are
trying
to
control.
For
example,
if
you
want
to
maintain
a
temperature
of
+
or
--
one
degree
then
we
typically
strive
for
at
least
ten times that or one-
tenth of a degree. If the analog input is a 12 bit
analog
input
and
the
temperature
range
for
the
sensor
is
0
to
400
degrees
then our
by
4,096
(12
bits)
=0.09765625
degrees.
[~]
We
say
because
it would
assume there was no noise and error in our
temperature sensor,
wiring,
and
analog
converter.
There
are
other
assumptions
such
as
linearity, etc.. The
point being--with 1/10 of a degree
accuracy--even with the usual amount of
noise and other problems-- one
degree
of accuracy should easily be attainable.
The
analog
output
is
often
simply
referred
to
as
Often
this
is
given
as
0~100
percent.
In
this
heating
example,
it
would
mean
the
valve
is totally closed (0%) or totally open
(100%).
The
setpoint
(SP)
is
simply--
what
process
value
do
you
want.
In
this
example--
what temperature do you want the process at?
The
PID
controller's
job
is
to
maintain
the
output
at
a
level
so
that
there
is
no
difference
(error)
between
the
process
variable
(PV)
and
the
setpoint (SP).
In Fig. 3, the valve could be
controlling the gas going to a heater,
the
chilling
of
a
cooler,
the
pressure
in
a
pipe,
the
flow
through
a
pipe,
the level in a tank, or any other
process control system. What the PID
controller is looking at is the
difference (or
and the SP.
SETPOINT
P
,
I
,
&D
CONSTANTS
Difference error
PID control
algorithm
process output
variable
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Fig .3 PIDcontrol
It looks at the
absolute error and the rate of change of error.
Absolute
error
means--is
there
a
big
difference
in
the
PV
and
SP
or
a
little
difference?
Rate
of
change
of
error
means--
is
the
difference
between
the
PV or
SP getting smaller or larger as time goes on.
When there is a
or
the
setpoint
quickly
changes--the
PID
controller
has
to
quickly
change
the
output
to
get
the
process
variable
back
equal
to
the
setpoint.
If
you
have a walk-in cooler
with a PID controller and someone opens the door
and
walks
in,
the
temperature
(process
variable)
could
rise
very
quickly.
Therefore the PID
controller has to increase the cooling (output) to
compensate for this rise in
temperature.
Once
the
PID
controller
has
the
process
variable
equal
to
the
setpoint,
a
good PID controller will not vary the output. You
want the output to
be very steady (not
changing) . If the valve (motor, or other control
element)
is
constantly
changing,
instead
of
maintaining
a
constant
value,
this could cause more wear on the
control element.
So
there
are
these
two
contradictory
goals.
Fast
response
(fast
change
in output) when there
is a
output) when the PV is close to
the setpoint.
Note that the output
often goes past (over shoots) the steady-state
output
to
get
the
process
back
to
the
setpoint.
For
example,
a
cooler
may
normally
have
its
cooling
valve
open
34%
to
maintain
zero
degrees
(after
the
cooler
has
been
closed
up
and
the
temperature
settled
down).
If
someone
opens the cooler, walks in, walks
around to find something, then walks
back
out,
and
then
closes
the
cooler
door--the
PID
controller
is
freaking
out because the
temperature may have raised 20 degrees! So it may
crank
the cooling valve open to 50, 75,
or even 100 percent--to hurry up and
cool the cooler back down--before
slowly closing the cooling valve back
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