Ziegler Nichols Tuning Method - Danfoss VLT AutomationDrive FC 300 Design Manual

FC 300 Design Guide
Optimisation of the process regulator
The basic settings have now been made; all that needs to be done is to optimise the proportional gain,
the integration time and the differentiation time (par. 7-33, 7-34, 7-35). In most processes, this can be
done by following the guidelines given below.
1. Start the motor
2. Set par. 7-33 (Proportional Gain) to 0.3 and increase it until the feedback signal again begins to
vary continuously. Then reduce the value until the feedback signal has stabilised. Now lower the
proportional gain by 40-60%.
3. Set par. 7-34 (Integration Time) to 20 sec. and reduce the value until the feedback signal again
begins to vary continuously. Increase the integration time until the feedback signal stabilises,
followed by an increase of 15-50%.
4. Only use par. 7-35 for very fast-acting systems only (differentiation time). The typical value is
four times the set integration time. The differentiator should only be used when the setting of
the proportional gain and the integration time has been fully optimised. Make sure that oscilla-
tions on the feedback signal is sufficiently dampened by the lowpass filter on the feedback signal.
NB!
If necessary, start/stop can be activated a number of times in order to provoke a variation of
the feedback signal.

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!
The method described must not be used on applications that could be damaged by the oscil-
lations created by marginally stable control settings.
The criteria for adjusting the parameters are
based on evaluating the system at the limit of sta-
bility rather than on taking a step response. We
increase the proportional gain until we observe
continuous oscillations (as measured on the feed-
back), that is, until the system becomes margin-
ally stable. The corresponding gain (K
the ultimate gain. The period of the oscillation
(P ) (called the ultimate period) is determined as
u
shown in Figure 1.
P should be measured when the amplitude of oscillation is quite small. Then we "back off" from this
u
gain again, as shown in Table 1.
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.
Introduction to FC 300
) is called
u
Proportional Gain
0.45 * K
u
0.6 * K
u
0.33 * K
u
®
MG.33.B8.02 - VLT is a registered Danfoss trademark
Figure 1: Marginally stable system
Integral Time
0.833 * P
u
0.5 * P
u
0.5 * P
u
Differentiation Time
-
0.125 * P
u
0.33 * P
u
37