Closed Loop Tuning

With some real processes the response to a stepchange or setpoint disturbance differs depending on the direction or size of the change. In this case it is irrelevent to look at the the open-loop response to tune the controller. Instead the closed-loop response ie the behaviour of the system with control, has to be studied.

The Zeigler Nichols Closed-Loop Tuning Method looks at the response of the system under proportional only control to obtain PI controller5 settings. The method is outlined below.

Set up the system with proportional only control and add a disturbance.
Alter the gain of the process until you obtain the smallest gain which gives constant amplitude oscillations. This gain is called the Ultimate Gain, .
Now evaluate the period of these constant oscillations. This is known as the Ultimate Period, P_u. Please refer to the diagram below.
Finally substitute these values into the table below to obtain the relevent controller parameters.

Controller Type Gain Reset Derivative

P /2 - -

PI /2.2 P_u/1.2 -

PID /1.7 P_u/2 P_u/8

Controller Type	Gain	Reset	Derivative
P	/2	-	-
PI	/2.2	P_u/1.2	-
PID	/1.7	P_u/2	P_u/8

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