Watlow 12LS Controller - Manual (Page 95)

5-4
There is no tuning step for the Output Filter. Adjusting the PID without the Filter(set at 0) will
give the fastest output response to a step change. The Filter may be turned on at any time. If, the
PV is cycling with the PB at 20% of SP or the output is changing more than 2-3% with good PID
values, the Digital Filter may be turned on. Settings of 2, 4, 6, 8, 10, 12, and 15 have been used.
Settings of 4, 6, and 10 are common filter settings. Increase filter setting in steps of 2 until output
or PV has stabilized. Remember to allow at least 20 minutes between adjustments.
After PID values are selected for proper response of the PV, these values will remain the same in
most processes without need of re-tuning. Most heat/cool processes do not have the
characteristics that require re-tuning of the PID constants.
The PB is one variable that would change when operating at a very low temperature and than
operating at a much higher temperature. For instance controlling at 250o F and 2250o F with the
same controller will require different PB values. See the PB Table on the PID Tuning Constants
page.
TI and TD will most likely remain the same without need of further tuning. There are two
situations that may require a new value for the TI. The first one is when changing process
material loads from a very large load to a much lighter load. The second situation is of a
changing process load due to exothermic based processes.
When changing TI and the Derivative is in use, the TD should also be changed.
Remember, most likely PID values will not need to be changed after they have been set correctly.
Changing PID values to correct for process problems, will normally cause more problems.
|-----------------------------------------------------|
¦ Table 6-1 PID Constants for ANAFAZE Systems ¦
|-----------------------------------------------------¦
¦ PID Mode ¦ Useful Range ¦ Nominal Setting ¦
|---------------|----------------|--------------------¦
¦ Proportional ¦ 10 to 200o F ¦ 40o F or 10% of ¦
¦ Band(PB) ¦ ¦ SP for any input ¦
|---------------|----------------|--------------------¦
¦ Integral(TI) ¦ 30 to 600 ¦ 60 seconds or ¦
¦ [Reset] ¦ Seconds ¦ [120, 180, 240] ¦
|---------------|----------------|--------------------¦
¦ Derivative(TD)¦ 3 to 60 ¦ OFF or 10 seconds ¦
¦ [Rate] ¦ Seconds ¦ [or 15% of TI] ¦
|---------------|----------------|--------------------¦
¦ Digital Filter¦ 0 to 25 ¦ 4 ¦
|---------------|----------------|--------------------|