National Instruments Xmath Interactive Control Design Module ICDM - Manual (Page 46)

Chapter 4 PID Synthesis
Xmath Interactive Control Design Module 4-6 ni.com
Derivative Term Normalization
The derivative term is low-frequency normalized, which means that at low
frequencies (below 1/T
diff
) it is nearly one, and so has little effect on the
overall controller transfer function at low frequencies. In particular, the
loop transfer function at s = 0 is not affected by the derivative term at all,
so static tracking, static actuator effort, and so on are not affected by the
derivative term. You can start by making the T
diff
term small and then
gradually increasing it until you get a good balance between better stability
margins and excessive actuator effort.
Rolloff Term Normalization
The two HF rolloff terms are low-frequency normalized, so they have little
effect at frequencies below T
hf
. You can start with small values for these
parameters, and then gradually increase them until you start to notice a
degradation in stability.