January 2012 6.16 The Control Loop 841
Feedback control
with following error
Following error (also known as servo lag) is a gap that remains between the
nominal position commanded by the NC and the actual position of the axis.
Simplified representation:
The nominal position value s
noml
for a given axis is compared with the actual
position value s
actl
and the resulting difference is the following error s
a
:
s
a
= s
noml
– s
actl
s
a
= following error
s
noml
= nominal position value
s
actl
= actual position value
The following error is multiplied by the k
v
factor and passed on as nominal
velocity value:
v = k
v
· s
a
v
noml
= nominal velocity value
The control loop gain, known as the k
v
factor, defines the amplification of the
position control loop. You must find the optimum k
v
factor by trial and error.
If you choose a k
v
factor that is too large, the following error will become very
small. However, this can lead to oscillations.
If you choose too small a k
v
factor, the axis will move to a new position too
slowly.
For axes that are interpolated with each other, the k
v
factors must be equal to
prevent contour deviations.
Define the k
v
factor in MP_kvFactor.
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