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.
Start the motor
1.
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%.
Set par. 7-34 (Integration Time) to 20 sec. and reduce the value until the feedback
3.
signal again begins to vary continuously. Increase the integration time until the feedback
signal stabilises, followed by an increase of 15-50%.
Only use par. 7-35 for very fast-acting systems only (differentiation time). The typical value is
4.
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 oscillations 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 and it can be considered quick and dirty.
NB!:
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
corresponding gain (called the ultimate gain) and
the period of the oscillation (also called the ultimate
period) are determined as shown in Figure 1.
P should be measured when the amplitude of oscillation is quite small. Then we "back
u
off" from this gain again, as shown in Table 1.
Type of Control
PI-control
PID tight control
PID some overshoot
Table 1: Ziegler Nichols tuning for regulator, based on a stability boundary.
Introduction to FC 300
Proportional Gain
0.45 * K
u
0.6 * K
u
0.33 * K
u
MG.33.B4.02 - 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
39