Marathon Computer AACC 2000 - Manual (Page 56)

Marathon Monitors Inc.
AACC 2000 Carbon Nov. 1, 1997
56
MANUAL TUNING
If for any reason automatic tuning gives unsatisfactory results, you can tune the controller
manually. There are a number of standard methods for manual tuning. The one
described here is the Ziegler-Nichols method.
With the process at its normal running process variable:
1. Set the Integral Time ‘
ti’ and the Derivative Time ‘td’ to OFF.
2. Set High Cutback and Low Cutback, ‘
Hcb’ and ‘Lcb’, to ‘Auto’.
3. Ignore the fact that the process variable may not settle precisely at the setpoint.
4. If the process variable is stable, reduce the proportional band ‘
Pb’ so that the process
variable just starts to oscillate. If the process variable is already oscillating, increase
the proportional band until it just stops oscillating. Allow enough time between each
adjustment for the loop to stabilise. Make a note of the proportional band value ‘B’
and the period of oscillation ‘T’.
5. Set the Pb, ti, td parameter values according to the calculations given in Table 4-2.
Type of control Proportional
band ‘Pb’
Integral time
‘ti’
Derivative time
‘td’
Proportional only 2xB OFF OFF
P + I control 2.2xB 0.8xT OFF
P + I + D control 1.7xB 0.5xT 0.12xT
Table 4-2 Tuning values