2.5.5 Optimisation of the Process Regulator
After configuring the basic settings as described in
chapter 2.5.5 Programming Order
, optimise the proportional
gain, the integration time and the differentiation time
(
7-33 Process PID Proportional Gain
,
7-34 Process PID Integral
Time
,
7-35 Process PID Differentiation Time
). In most
processes, complete the following procedure:
1.
Start the motor.
2.
Set
7-33 Process PID Proportional Gain
to 0.3 and
increase it until the feedback signal again begins
to vary continuously. Reduce the value until the
feedback signal has stabilised. Lower the propor-
tional gain by 40-60%.
3.
Set
7-34 Process PID Integral Time
to 20 s 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
7-35 Process PID Differentiation Time
for
fast-acting systems only (differentiation time). The
typical value is 4 times the set integration time.
Use the differentiator 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 lowpass filter on the feedback signal.
NOTICE
If necessary, start/stop can be activated a number of
times in order to provoke a variation of the feedback
signal.
2.5.6 Ziegler Nichols Tuning Method
To tune the PID controls of the frequency converter,
Danfoss recommends the Ziegler Nichols tuning method.
NOTICE
Do not use the Ziegler Nichols Tuning method in
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. Increase the proportional gain
until observing continuous oscillations (as measured on
the feedback), that is, until the system becomes marginally
stable. The corresponding gain
(K
u
)
is called the ultimate
gain, and is the gain at which the oscillation is obtained.
The period of the oscillation
(P
u
)
(called the ultimate
period) is determined as shown in
and
should be measured when the amplitude of oscillation is
quite small.
1.
Select only proportional control, meaning that
the integral time is set to the maximum value,
while the differentiation time is set to 0.
2.
Increase the value of the proportional gain until
the point of instability is reached (sustained
oscillations) and the critical value of gain,
K
u
, is
reached.
3.
Measure the period of oscillation to obtain the
critical time constant,
P
u
.
4.
Use
to calculate the necessary PID
control parameters.
The process operator can do the final tuning of the control
iteratively to yield satisfactory control.
130BA183.10
y(t)
t
Pu
Illustration 2.28 Marginally Stable System
Type of
control
Proportional
gain
Integral time
Differentiation
time
PI-control
0.45 *
K
u
0.833 *
P
u
–
PID tight
control
0.6 *
K
u
0.5 *
P
u
0.125 *
P
u
PID some
overshoot
0.33 *
K
u
0.5 *
P
u
0.33 *
P
u
Table 2.10 Ziegler Nichols Tuning for Regulator
Product Overview
Design Guide
36
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MG06B402
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