Danfoss VLT AQUA Drive Design Manual page 27

VLT ® AQUA Drive Design Guide
2.9.6. Manual PID Adjustment
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2.9.7. Ziegler Nichols Tuning Method
In general, the above procedure is sufficient for water applications. However, other, more so-
phisticated procedures can also be used. The Ziegler Nichols tuning method is a technique that
was developed in the 1940s and is still commonly used today. It generally provides acceptable
control performance using a simple experiment and parameter calculation.
2.1:
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3.
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 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 low-pass filter for the feedback signal (par 6 16, 6 26, 5 54 or 5 59, as
required).
NOTE
This method must not be used on applications that could be damaged by oscillations
created by marginally stable control settings.
Figure 1: Marginally stable system
Select proportional control only. That is, PID Integral Time (par. 20-94) is set to Off
(10,000 s), and the PID Differentiation Time (par. 20-95) is also set to Off (0 s in this
case).
Increase the value of the PID Proportional Gain (par 20-93) until the point of instability
is reached, as indicated by sustained oscillations of the feedback signal. The PID Pro-
portional Gain that causes sustained oscillations is called the critical gain, K
Measure the period of oscillation, P
MG.20.N2.22 - VLT ® is a registered Danfoss trademark.
2. Introduction to the VLT AQUA Drive
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