7.2.8 Manual Tuning

 

If for any reason automatic tuning gives unsatisfactory results, or if you prefer, 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.

 

Adjust the setpoint to its normal running conditions (it is assumed this will be above the PV so that heat only is applied)

 

Set the Integral Time ‘Ti’ and the Derivative Time ‘TD’ to ‘OFF’.

 

Set High Cutback ‘cBHi’ and Low Cutback ‘cBLo’ to ‘Auto’.

 

Ignore the fact that the PV may not settle precisely at the setpoint.

 

If the PV is stable, reduce the proportional band so that the PV just starts to oscillate. Allow enough time between each adjustment for the loop to stabilise. Make a note of the proportional band value ‘PB’ and the period of oscillation ‘T’. If PV is already oscillating measure the period of oscillation ‘T’, then increase the proportional band until it just stops oscillating. Make a note of the value of the proportional band at this point.

 

Set the proportional band, integral time and derivative time parameter values according to the calculations given in the table below:-

 

Type of control	Proportional band (PB)	Integral time (TI) seconds	Derivative time (TD) seconds
Proportional only	2x PB	OFF	OFF
P + I control	2.2x PB	0.8xT	OFF
P + I + D control	1.7x PB	0.5xT	0.12xT