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Tune algorithm:
The Tune procedure is activated by selecting Yes on Automatic Tune selection screen
The Tune can select two types of procedures:
1) The Pre-TUNE algorithm
2) The Self-Tune algorithm
1) Pre-TUNE ALGORITHM
To implement the Pre-TUNE algorithm set the instrument in manual mode and then
select Run "Pre-Tune". "Pre-Tune" will switch to OFF after PID parameters calculation
(during this procedure the LED will be flashing).
The basic concepts of auto tuning system are based on the open loop pulse response,
for this reason the Pre-TUNE function may be activated only in manual mode.
The equivalent mathematical model of the process is characterized by three
parameters: the gain, the time constant and the equivalent time delay.
The power output of the controller is changed by a small step value (This value has to
be entered by the user before the pre tune can take place in manual mode) and the
controller stores the process variable response. If the pre-tune step is set too high it
will create an overshoot and therefore needs to be reduced. The pre-tune step power
is added to manual power and displayed to the user but is not changeable. The
expected range is from –25.0% to 25.0% of full scale. The default value is 10.0%.
2) Self-Tune ALGORITHM
In order to implement the Self-Tune algorithm the instrument should be in automatic
mode. Then select run “Self-Tune”. In this case the “ON” will be remembered by the
instrument even if the instrument was switched off. In order to deactivate the
Self-Tune, it is necessary to return the “Self-Tune” parameter to “No”
The self-tune is a on-line algorithm that “observes” the measured value and looks for
oscillation due to variations of the load or set-point changes. When a
significant pattern is “recognized” a decisional procedure starts in order to
recalculate the parameters of PID controller.
When the Self-tune procedure is enabled the PI parameters (PB, TI) are read only.
Automatic stand-by:
This function avoids overshoot due to temporary process interruptions (PV goes to
zero). When the input signal goes to zero the controller output reaches the
saturation for integral factor effect; when the process restarts the controlled output
will have an excessive and dangerous overshoot.
