PID for process regulation, cont.
Inverse regulation
Normal regulation means that the motor speed increa-
ses when the reference/setpoint is higher than the
feedback signal. If there is a need for inverse regula-
tion, in which the speed is reduced when the reference/
setpoint is higher than the feedback signal, inverse
must be programmed in parameter 420
PID normal/
inverse control
.
Anti Windup
The process regulator is factory preset with an active
anti-windup function. This function ensures that when
either a frequency limit, current limit or voltage limit
reached, the integrator will be initialised for a frequen-
cy that corresponds to the present output frequency.
This avoids integration on a deviation between the ref-
erence/setpoint and the actual state the process, the
controller of which is not possible means of a speed
change. This function can be disabled in parameter
421
PID anti windup
.
Start-up conditions
In some applications, optimum setting of the process
regulator will mean that it takes an excessive time for
the required process state to be reached. In such ap-
plications it might be an advantage to fix an output
frequency to which the frequency converter is to bring
the motor before the process regulator is activated.
This is done by programming a
PID start-up frequen-
cy
in parameter 422.
Differentiator gain limit
If there are very quick variations in a given application
with respect to the reference/setpoint signal or the
feedback signal, the deviation between reference/set-
point and the actual process state will quickly change.
The differentiator may thus become too dominant. This
is because it reacts to the deviation between the ref-
erence/setpoint and the actual process state. The
quicker the deviation changes, the stronger the result-
ing differentiator frequency contribution. The differen-
tiator frequency contribution can thus be limited to
allow the setting of a reasonable differentiation time for
slow changes and a suitable frequency contribution for
quick changes. This is done in parameter 426,
PID Dif-
ferentiator gain limit
.
Lowpass filter
If there are ripple currents/voltages on the feedback
signal, these can be dampened by means of a built-in
lowpass filter. Set a suitable lowpass filter time con-
stant. This time constant represents the limit frequency
of the ripples occurring on the feedback signal. If the
lowpass filter has been set to 0.1s, the limit frequency
will be 10 RAD/sec., corresponding to (10/ 2 x
π
) = 1.6
Hz. This means that all currents/voltages that vary by
more than 1.6 oscillations per second will be removed
by the filter.
In other words, regulation will only be carried out on a
feedback signal that varies by a frequency of less than
1.6 Hz. Choose a suitable time constant in parameter
427,
PID Lowpass filter time
.
Optimisation of the process regulator
The basic settings have now been made; all that re-
mains to be done is to optimise the proportional gain,
the integration time and the differentiation time (pa-
rameters 423, 424 and 425). In most processes, this
can be done by following the guidelines given below.
1.
Start the motor.
2.
Set parameter 423
PID proportional gain
to
0.3 and increase it until the process shows
that the feedback signal is unstable. Then re-
duce the value until the feedback signal has
stabilised. Now lower the proportional gain by
40-60%.
3.
Set parameter 424
PID integration time
to 20
s and reduce the value until the process
shows that the feedback signal is unstable.
Increase the inte-gration time until the feed-
back signal stabilises, followed by an in-
crease of 15-50%.
4.
Parameter 425
PID differentiation time
is only
used in very fast-acting systems. The typical
value is 1/4 of the value set in parameter 424
PID Integration time
. The differentiator should
only be used when the setting of the propor-
tional gain and the integra-tion time have
been fully optimised.
NB!
If necessary, start/stop can be activated a
number of times in order to provoke an
unstable feedback signal.
VLT
®
6000 HVAC Series
= factory setting, () = display text, [] = value for use in communication via serial communication port
MG.61.B5.02 - VLT
®
is a registered Danfoss trademark
147
Pr
ogr
a
mmi
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