Illustration 2 - Danfoss VLT Series Design Manual

2. Introduction to VLT HVAC Drive
Once the drive's Closed Loop Controller has
been set up, the performance of the controller
should be tested. In many cases, its perform-
ance may be acceptable using the default val-
2
ues of PID Proportional Gain (par. 20-93) and
1.
2.
3.
4.
2.9.2. Ziegler Nichols Tuning Method
In general, the above procedure is sufficient for HVAC applications. However, other, more so-
phisticated procedures can also be used. The Ziegler Nichols tuning method is a technique which
was developed in the 1940s, but is still commonly used today. It generally provides acceptable
control performance using a simple experiment and parameter calculation.

Illustration 2.1:

system
1.
2.
36
Start the motor
Set par. 20-93 (PID Proportional Gain) to 0.3 and increase it until the feedback signal
begins to oscillate. If necessary, start and stop the drive or make step changes in the
setpoint reference to attempt to cause oscillation. Next reduce the PID Proportional Gain
until the feedback signal stabilizes. Then reduce the proportional gain by 40-60%.
Set par. 20-94 (PID Integral Time) to 20 sec. and reduce it until the feedback signal
begins to oscillate. If necessary, start and stop the drive or make step changes in the
setpoint reference to attempt to cause oscillation. Next, increase the PID Integral Time
until the feedback signal stabilizes. Then increase of the Integral Time by 15-50%.
Par. 20-95 (PID Differentiation Time) should only be used for very fast-acting systems.
The typical value is 25% of the PID Integral Time (par. 20-94). The differentiator should
only be used when the setting of the proportional gain and the integral time has been
fully optimized. Make sure that oscillations of the feedback signal are sufficiently damp-
ened by the lowpass filter for the feedback signal (par 6 16, 6 26, 5 54 or 5 59, as
required).
NB!
This method must not be used on applications that could be damaged by oscillations
created by marginally stable control settings.
Figure 1: Marginally stable
Select proportional control only. That
is, PID Integral Time (par. 20-94) is
set to Off (10000 s) and PID Differ-
entiation Time (par. 20 95) is also set
to Off (0 s, in this case).
Increase the value of the PID Pro-
portional Gain (par 20-93) until the
MG.11.B2.02 - VLT
PID Integral Time (par. 20-94). However, in
some cases it may be helpful to optimize these
parameter values to provide faster system re-
sponse while still controlling speed overshoot.
In many situations, this can be done by fol-
lowing the procedure below.
point of instability is reached, as in-
dicated by sustained oscillations of
the feedback signal. The PID Propor-
tional Gain that causes sustained os-
cillations is called the critical gain,
K .
u
3. Measure the period of oscillation,
P .
u
NOTE: P should be measured when
u
the amplitude of oscillation is rela-
tively small. The output must not sat-
urate (i.e., the maximum or mini-
mum feedback signal must not be
reached during the test).
4. Use the table below to calculate the
necessary PID control parameters.
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VLT HVAC Drive Design Guide