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9.4 WATLOW Series 988 User’s Manual
Enhanced Software Features, Chapter 9
Enhanced Software
The outer loop can be auto-tuned by setting [`AUt] to [PidA]. While auto-
tuning, the outer loop will be controlled in an on/off mode at a set point
equal to [AtSP] x [`SP1]. In a heating application, make sure the set
point is set at a value above ambient temperature. In most cases, the
auto-tuning feature will tune [PidA] for acceptable control. If not, you
must then manually tune the outer loop.
Before beginning manual tuning, record the values of [Pb1A] and [rE1A]
generated by the auto-tuning feature. The auto-tune for the outer loop will
not generate a value for [rA1A], because rate (derivative) in the outer loop
seems to cause instability in most systems.
Start manual tuning by setting [rE1A] to [`)00]. Enter the desired
process set point and let the system stabilize. Once the system stabilizes,
observe the value of [`Pr2] in the Display Menu. If the [`Pr2] value fluc-
tuates, make the proportional band setting [Pb1A] wider until the [`Pr2]
value stabilizes. Make adjustment [Pb1A] in 5° to 10° increments, allowing
time between adjustments for the system to stabilize.
Once [`Pr2] has stabilized, observe percent power in the display loop. It
should be stable, ±10%. At this point, the process temperature should also
be stable, but will exhibit droop (stabilized below set point). The droop can
be eliminated with reset of integral.
Start with a setting of 0.01; allow 10 minutes for the process temperature
to come up to set point. If it has not, increase the setting to 0.05 and wait
another 10 minutes. After this, double the reset setting until process value
equals the set point. If the process becomes unstable, the reset value is
too large. Decrease the setting until the process stabilizes.