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When KP and Ti are both properly tuned, the controller
Command signal will produce heat output that quickly
and smoothly reaches the desired setpoint.
When KP is set too high, the controller Command signal
responds too aggressively, causing the the heat output to
overshoot the desired setpoint.
When KP is set too low, the controller Command signal
is not aggressive enough and the heat output rises too slowly.
DESIRED TEMPERATURE (SETPOINT)
DESIRED TEMPERATURE (SETPOINT)
DESIRED TEMPERATURE (SETPOINT)
EXISTING TEMPERATURE
“ERROR”
EXISTING TEMPERATURE
“ERROR”
EXISTING TEMPERATURE
“ERROR”
FIGURE 15.1.0 KP SETPOINT CHARTS
15.1.2 Time Integral (Ti)
If the KP setting did not have some means of control, the resulting command signals would
constantly over-shoot the setpoint. The Time Integral causes the controller to re-examine the
amount of error at specific time intervals to see the amount of error remaining. Ti produces a
damping effect on the KP value to reduce over-shooting the setpoint. Ti is an actual measure of
time so that when the value is reduced, the frequency of sampling increases.
Example:
the default Ti value is 30. If the value were changed to 15, sampling would occur
twice as often. If the value were changed from 30 to 60, sampling would occur only half as
often. If Ti were changed to 500, the result is that the sampling value would be insignificant and
corrections to KP would not be happening.
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If the Ti value is set too low (meaning that sampling is occurring too often), the increase
in the command signal will immediately begin to slow and keep getting slower because Ti is
damping the KP value too much.
u
If Ti is set too high (sampling is not occurring often enough), the Command signal will
overshoot the setpoint in both a negative and positive direction, causing short-cycling of the
heating device.
The process of adjusting KP and Ti to provide the most satisfactory controller output is known
as tuning the controller. There are some sophisticated math equations that can be done to
predict where the KP and Ti settings should be, but in the field, adjustment is normally done by
a trial-and-error method. The KP setting is first adjusted and then the Ti setting is adjusted to
manage the KP setting.
Tools needed:
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Laptop computer to view the Heating screens and make adjustments to KP and Ti.
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A stopwatch or watch with a second hand to monitor times.
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Connect the laptop to the controller via ethernet.
REFERENCE
