Optimisation Of The Process Regulator; Ziegler Nichols Tuning Method - Danfoss VLT AutomationDrive FC 300 Design Manual

Product Introduction
3.4.10 Optimisation of the Process
Regulator
After the basic settings have been made, optimise the
following:
•
Proportional gain
•
Integration time
•
Differentiation time
In most processes, this can be done by following these
steps:
1. Start the motor.
2. Set 7-33 Process PID Proportional Gain to 0.3 and
increase it until the feedback signal begins to
vary continuously. Then reduce the value until
the feedback signal has stabilised. Now lower the
proportional gain by 40-60%.
3. Set 7-34 Process PID Integral Time to 20 s and
reduce the value until the feedback signal begins
to vary continuously. Increase the integration
time until the feedback signal stabilises, followed
by an increase of 15-50%.
4. Only use 7-35 Process PID Differentiation Time for
very fast-acting systems only (differentiation
time). The typical value is 4 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 are sufficiently dampened by the
low-pass filter on the feedback signal.
NOTICE
If necessary, start/stop can be activated a number of
times to provoke a variation of the feedback signal.

3.4.11 Ziegler Nichols Tuning Method

Several tuning methods can be used to tune the PID
controls of the frequency converter. One approach is to
use the Ziegler Nichols tuning method.
NOTICE
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. The proportional gain is
increased until continuous oscillations are observed via the
feedback, meaning the system is marginally stable. The
®
VLT AutomationDrive FC 300 Design Guide 90-1200 kW
period of the oscillation (P
Illustration 3.25.
y(t)
Illustration 3.25 Marginally Stable System
Measure P
small. Then "back off" from this gain again, as shown in
Table 3.12.
K is the gain at which the oscillation is obtained.
u
Type of
control
PI-control
PID tight
control
PID some
overshoot
Table 3.12 Ziegler Nichols Tuning for Regulator,
Based on a Stability Boundary
According to the Ziegler Nichols rule, experience has
shown that the control setting described in the steps
below provides a good closed loop response for many
systems. The process operator can do the final tuning of
the control repeatedly to yield satisfactory control.
Step-by-Step Description
1.
2.
3.
4.
MG34S202 - Rev. 2013-08-19
) is determined as shown in
u
P u
when the amplitude of oscillation is quite
u
Proportional Integral time
gain
0.45 * K 0.833 * P
u u
0.6 * K 0.5 * P
u u
0.33 * K 0.5 * P
u u
Select only Proportional Control (Integral time is
selected to the maximum value, while the differ-
entiation time is selected to zero).
Increase the value of the proportional gain until
the point of instability is reached (sustained
oscillations) and the critical value of gain, K
reached.
Measure the period of oscillation to obtain the
critical time constant, P .
u
Use Table 3.12 to calculate the necessary PID
control parameters.
3 3
t
Differentiation
time
-
0.125 * P
u
0.33 * P
u
, is
u
41