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PMI Controller • 185 • Chapter 9 Features
Once the Process Value has been at set point for a suitable period (about 30 minutes for a
fast process to roughly two hours for a slower process) and if no further tuning of the PID
parameters is desired or needed, TRU-TUNE+™ may be turned off. However, keeping the
controller in the adaptive mode allows it to automatically adjust to load changes and com-
pensate for differing control characteristics at various set points for processes that are not
entirely linear.
Once the PID parameters have been set by the TRU-TUNE+ adaptive algorithm, the process,
if shut down for any reason, can be restarted in the adaptive control mode. Turn TRU-TUNE+
on or off with TRU-TUNE+ Enable t.tUn (Setup Page, Loop Menu).
Use TRU-TUNE+ Band tbnd (Setup Page, Loop Menu) to set the range above and below the
set point in which adaptive tuning will be active. Adjust this parameter only in the unlikely
event that the controller is unable to stabilize at the set point with TRU-TUNE+ Band set to
auto (0). This may occur with very fast processes. In that case, set TRU-TUNE+ Band to a
large value, such as 100.
Use TRU-TUNE+ Gain t.gn (Setup Page, Loop Menu) to adjust the responsiveness of the adap-
tive tuning calculations. Six settings range from 1, with the most aggressive response and
most potential overshoot (highest gain), to 6, with the least aggressive response and least
potential for overshoot (lowest gain). The default setting, 3, is recommended for loops with
thermocouple feedback and moderate response and overshoot potential.
Before Tuning
Before autotuning, the controller hardware must be installed correctly, and these basic con-
guration parameters must be set:
• Sensor Type sEn (Setup Page, Analog Input Menu), and scaling, if required;
• Function fn (Setup Page, Output Menu) and scaling, if required.
How to Autotune a Loop
1. Enter the desired set point or one that is in the middle of the expected range of set
points that you want to tune for.
2. Initiate an autotune. (See Autotuning in this chapter.)
Note:
Enable TRU-TUNE+ only after autotune is complete. It should be disabled before autotune
is initiated.
When autotuning is complete, the PID parameters should provide good control. As long as the
loop is in the adaptive control mode, TRU-TUNE+ continuously tunes to provide the best pos-
sible PID control for the process.
WARNING!
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During autotuning, the controller sets the output to 100 percent and attempts to drive
the Process Value toward the set point. Enter a set point and heat and cool power limits
that are within the safe operating limits of your system.
Inputs
Calibration Offset
Calibration offset allows a device to compensate for an inaccurate sensor, lead resistance or
other factors that affect the input value. A positive offset increases the input value, and a
negative offset decreases the input value. The input offset value can be viewed or changed
with Calibration Offset i.Ca (Operations Page, Analog Input Menu).
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