Ziegler Nichols Tuning Method - GE AF-650 GP Design Manual

3.4.5 Ziegler Nichols Tuning Method

In order to tune the PID controls of the frequency converter, several tuning methods can be used. One approach is to use a technique which was developed in
the 1950s but which has stood the test of time and is still used today. This method is known as the Ziegler Nichols tuning method.
NB!
3
The method described must not be used on applications that could be damaged by the oscillations created by marginally stable control settings.
The criteria for adjusting the parameters are based on evaluating the system
at the limit of stability rather than on taking a step response. We increase the
proportional gain until we observe continuous oscillations (as measured on
the feedback), that is, until the system becomes marginally stable. The cor-
responding gain (K ) is called the ultimate gain. The period of the oscillation
u
(P ) (called the ultimate period) is determined as shown in Figure 1.
u
P should be measured when the amplitude of oscillation is quite small. Then we "back off" from this gain again, as shown in Table 1.
u
K is the gain at which the oscillation is obtained.
u
Type of Control
PI-control
PID tight control
PID some overshoot
Table 1: Ziegler Nichols tuning for regulator, based on a stability boundary.
Experience has shown that the control setting according to Ziegler Nichols rule provides a good closed loop response for many systems. The process operator
can do the final tuning of the control iteratively to yield satisfactory control.
Step-by-step Description:
Step 1: Select only Proportional Control, meaning that the Integral time is selected to the maximum value, while the differentiation time is selected to zero.
Step 2: Increase the value of the proportional gain until the point of instability is reached (sustained oscillations) and the critical value of gain, K
Step 3: Measure the period of oscillation to obtain the critical time constant, P
Step 4: Use the table above to calculate the necessary PID control parameters.
34
Proportional Gain
0.45 * K
u
0.6 * K
u
0.33 * K
u
.
u
Illustration 3.2: Figure 1: Marginally stable system
Integral Time
0.833 * P
u
0.5 * P
u
0.5 * P
u
AF-650 GP Design Guide
Differentiation Time
-
0.125 * P
u
0.33 * P
u
, is reached.
u