Thermcraft XST-3-0-36-3V - Manual (Page 76)

Tuning Installation and Operation Handbook
4-4 2408 and 2404 Controller
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 temperature:
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 temperature may not settle precisely at the setpoint.
4. If the temperature is stable, reduce the proportional band ‘
Pb’ so that the temperature just
starts to oscillate. If the temperature 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