Watlow Series 998 - Manual (Page 103)

WATLOW Series 998 User’s Manual 7.1 Tuning, Alarms and Error Codes, Chapter 7
Auto-tuning (Heat and/or Cool)
The Series 998 can automatically tune the PID parameters for both
channel A and channel B to fit the characteristics of your particular sys-
tem. Only one channel’s PID set can be auto-tuned at a time.
Before beginning the auto-tune sequence, make sure the [AtSP] parame-
ter located in the Global Menu is at the proper setting. This allows the
user to select the tuning set point as a percentage of the current control
set point. See Chapter 4 for more information on this parameter. The fig-
ure below uses the 90% default setting to define the auto-tuning process.
Once the auto-tune sequence has begun, the control outputs proportional
band for the associated channel is set to 0 and that channel goes into an
on/off mode of control at the channel’s set point percentage determined
by the [AtSP] parameter. The channel’s displayed set point remains
unchanged. Note: Any change in the set point while in auto-tune restarts
the auto-tune procedure.
Once the control finishes “learning” the system, it returns the channel to
standard PID control with the PID values automatically set as a result of
auto-tuning.
Once auto-tune has begun, the process must cross the set point four
times within 80 minutes for the 998 to successfully complete the auto-
tune. If this does not happen within the time limit, the Series 998 chooses
PID values based on the 80-minute tuning cycle performed.
Figure 7.1 -
Auto-tuning
Example.
200 °
Temp
Set Point
100 °
Process
180 °
Auto-tune
Begins
Auto-tune
Complete
90% of
Set Point
[AtSP] Default Setting
Auto-tuning at a Set Point of 200 ° F
Time
Chapter 7 Tuning, Manual
Operation, Alarms and Error
Codes