(IV.83)

or, Suppose there exists a modeling error of 50% in estimating the dead time of the process and the true value of the dead time is 0.45 instead of 0.3,then the phase lag encountered by the process would be

(IV.84)

which is 2° more than the safety limit for stability. Hence, the phase margin of is not sufficient for handling 50% error in dead time estimation. It is left to the reader to verify that a phase margin of 45° will suffice for handling 50% error in dead time estimation.

 

IV.5.4 Ziegler Nichols Tuning technique

Unlike process reaction curve method which uses open loop response data, Ziegler Nichols tuning technique uses closed loop response data. The following settings are given by this technique for feedback controllers:

Table IV.6: Controller settings through Ziegler-Nichols tuning technique
Controller
P
PI
PID