Setting rules for PID Controller

Setting rules to Ziegler/Nichols

Right at the initial stage of modern control technology, J.G. Ziegler and N.B. Nichols have specified setting rules (Picture 1), which are still widely used today. These are intended for cases where no model (or inflectional tangent model) of the controlled system is available and the closed control loop can be operated safely along the stability limit.

These rules are as follows:

1. Set the controller as a P-controller (Tv = 0 Tn = ...).
2. The amplitude factor KR of the controller is increased until the closed control loop is on the point of performing unattenuated oscillations (stability limit). This determines the critical amplitude factor KRk and the period of oscillation Tk of this sustained oscillation.
3. Based on these two parameters (KRk, Tk), the controller parameters KR, Tn and Tv are then to be calculated as per controller type according to the following specification and set on the computer.

Picture 1: Setting rules to Ziegler/Nichols


However, experience shows that these setting values only lead to workable closed control loop behaviour, if the ratio of transient time Tg to time delay Tu of the controlled system is not too great, i.e. the system in the model of the transient function shows a noticeable time delay.

Setting rules to Chien/Hrones/Reswick

If we are dealing with an inflectional tangent model of the controlled system, then the setting rules of Chien, Hrones and Reswick are to be used. The setting rules for this are shown in the following table (Picture 2).

Picture 2: Setting rules to Chien/Hrones/Reswick


For I-controlled systems the expression:

1/(KIS * Tu)

is to be used instead of:

Tg/(KS * Tu).

These rules can also be applied to a total time constant model, provided Tu = Tt and Tg = T are set.