Transcript of "PROCESS CONTROLLER for temperature, flow, pressure etc"
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PROCESS
CONTROLLER
for temperature, flow,
pressure etc
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How do Temperature Controllers work?
• A temperature controller gets input from
temperature sensor such as a
thermocouple or RTD.
• It compares the actual temperature to the
desired control temperature, or set point,
• and provides an output in the form of
transistor output or relay
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What Are the Different Types of Controllers
There are three basic types of
controllers:
• on-off,
• proportional
• PID.
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on-off controller is the simplest form of temperature control
device.
• The output from it is either ON or OFF, with no middle state.
• It will switch the output only when the temperature crosses the
set point.
• For heating control, the output is ON when the temperature is
below the set point, and OFF above set point.
• Since the temperature crosses the set point to change the
output state, the process temperature will be cycling
continually, going from below set point to above, and back
below.
• On-off control is usually used where a precise control is
not necessary, where the mass of the system is so great that
temperatures change extremely slowly, or for a temperature
alarm.
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Proportional Control
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Proportional controls are designed to eliminate the cycling associated with
on-off control.
A proportional controller decreases the average power supplied to the
heater as the temperature approaches set point.
This has the effect of slowing down the heater so that it will not overshoot
the set point, but will approach the set point slowly and maintain a stable
temperature.
This proportioning action can be accomplished by turning the output ON and
OFF for short time intervals.
This “time proportioning” varies the ratio of “on” time to “off” time to control
the temperature.
The proportioning action occurs within a “proportional band” around the
set point temperature. Outside this band, the controller functions as an onoff unit, with the output either fully on (below the band) or fully off (above the
band). However, within the band, the output is turned on and off in the ratio
of the measurement difference from the set point.
At the set point (the midpoint of the proportional band), the output on:-off
ratio is 1:1; that is, the on-time and off-time are equal. if the temperature is
further from the set point, the on- and off-times vary in proportion to the
temperature difference.
If the temperature is below set point, the output will be ON longer; if the
temperature is too high, the output will be OFF longer.
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PID Control
• PID controller provides proportional action with l with
two additional adjustments, integral and derivative, which
helps the unit automatically compensate for changes in
the system.
• Integral and derivative, are expressed in time-based
units; they are also referred as RESET and RATE.
• The proportional, integral and derivative terms must be
individually adjusted or “tuned” to a particular system
using trial and error.
• It provides the most accurate and stable control of the
three controller types, and is best used in systems which
have a relatively small mass and those which react
quickly to changes in the energy added to the process.
• It is recommended in systems where the load changes
often and the controller is expected to compensate
automatically the amount of energy available, or the
mass to be controlled.
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PID CONTROLLER
PID controllers are process controllers with the
following characteristics:
• Continuous process control Analog input
(also known as "measurement" or "Process
Variable" or "PV")
• Analog output
• (referred to simply as "output")
• Set point (SP)
• Proportional (P), Integral (I), and / or Derivative
(D) constants
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• Once the PID controller has the
process variable equal to the set
point, a good PID controller will not
vary the output.
• It is desired to maintain the output
very steady (not changing).
• If the valve (motor, or other control
element) are constantly changing,
instead of maintaining a constant
value, this could case more wear on
the control element.
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PID CONTROLLER
• Examples of "continuous process control"
are temperature, pressure, flow, and level
control.
• PID controller functionality is a common
feature of programmable logic controllers
(PLC). Software PID loops are the most
stable, because they do not wear out as
compared to mechanical control systems.
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• Proportional Band is referred to as
Gain
• Integral Band is referred to as
Reset
• Derivative Band is referred to as
Rate
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PID CONTROLLER
• The analog output is often simply referred
to as "output“ and is given as 0 to 100%.
(In this heating example, it would mean is
the valve totally closed,(0 %) or totally
open (100 %).
• The set point (SP) is simply -- what
process value do you want.
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• Set point - the desired value of the controlled variable X
• Error - control error. This is the difference between the set
point and the measured real controlled value.
• Y - the controller output
• V - delayed controller output ( the Delay and Process blocks
form the model of the controlled process)
• X - controlled value. This is the output of the controlled
process, for example temperature, pressure, motor velocity,
flow etc.
• Xm - the measurement result. The measurement instrument
may have the gain different from zero and a first order
inertia, given by its time constant.
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In the diagram 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.
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PID CONTROLLER
• So there are these two
contradictory goals.
• Fast response (fast change in
output) when there is a "process
upset",
• but slow response (steady
output) when the PV is close to
the set point
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• When there is a "process upset",
meaning, when the process variable OR the
set point quickly changes -- the PID
controller has to quickly change the output to
get the process variable back equal to the
set point. For example in a walk-in
cooler with a PID controller 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.
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PID CONTROLLER
1. What the PID controller is looking at is the
difference (or "error") between the PV and the
SP.
2. 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.
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PID CONTROLLER- Explanation
• Note that the output often goes past (over shoots) the
steady-state output to get the process back to the set
point.
• For example, a cooler may normally have it's 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
down to 34 percent.
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PID CONTROLLER
GAS HEATER EXAMPLE
• The PID controller would receive as input 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 set point. Instead of the
temperature bouncing back and forth between two
points, the temperature is held steady.
• If the set point is lowered, then the PID controller
automatically reduces the amount of gas flowing to the
heater.
• If the set point is raised, then the PID controller
automatically increases the amount of gas flowing to the
heater.
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PID CONTROLLER
Accuracy Explanation
• The analog input (measurement) is called the "process
variable" or "PV". You want the PV to be a highly
accurate indication of the process 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 "theoretical"
accuracy is calculated to be 400 degrees divided by
4,096 (12 bits) = 0.09765625 degrees. We say
"theoretical" because it would assume there was no
noise and error in our temperature sensor, wiring, and
analog converter -- even with the usual amount of noise
and other problems -- one degree of accuracy should
easily be attainable.
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