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4 Servo Adjustment
MITSUBISHI CNC
(2) Setting the speed loop lead compensation
The speed loop lead compensation (SV008: VIA) determines the characteristics of the speed loop
mainly at low frequency regions. 1364 is set as a standard, and 1900 is set as a standard during SHG
control. The standard value may drop in respect to loads with a large inertia.
When the VGN1 is set lower than the standard value because the load inertia is large or because
machine resonance occurred, the speed loop control band is lowered. If the standard value is set in the
leading compensation in this status, the leading compensation control itself will induce vibration. In
concrete terms, a vibration of 10 to 20Hz could be caused during acceleration/ deceleration or stopping,
and the position droop waveform could be disturbed when accelerating to a constant speed and when
stopped. (Refer to lower left drawing)
This vibration cannot be suppressed by the vibration suppression functions. Lower the VIA in
increments of 100 from the standard setting value. Set a value where vibration does not occur and the
position droop waveform converges smoothly. Because lowering the VIA causes a drop in the position
control's trackability, the vibration suppression is improved even when a disturbance observer is used
without lowering the VIA. (Be careful of machine resonance occurrence at this time.)
If VIA is lowered, the position droop waveform becomes smooth and overshooting does not occur.
However, because the trackability in respect to the position commands becomes worse, the positioning
time and accuracy are sacrificed. VIA must be kept high (set the standard value) to guarantee precision,
especially in high-speed contour cutting (generally F = 1000 or higher). In other words, in a machine
aiming for high speed and high accuracy, a large enough value must be set in VGN1 so that VIA does
not need to be lowered. When adjusting, the cutting precision will be better if adjustment is carried out to
a degree where overshooting does not occur and a high VIA is maintained, without pursuing position
droop smoothness.
If there are no vibration or overshooting problems, the high-speed contour cutting precision can be
further improved by setting the VIA higher than the standard value. In this case, adjust by raising the VIA
in increments of 100 from the standard value.
Setting a higher VIA improves the trackability regarding position commands in machines for which cycle
time is important, and the time to when the position droop converges on the in-position width is
shortened.
It is easier to adjust the VIA to improve precision and cycle time if a large value (a value near the
standard value) can be set in VGN1, or if VGN1 can be raised equivalently using the disturbance
observer.
Vibration waveform with lead com
p
ensation control Ad
j
usted
p
osition droo
p
waveform
S
p
eed FB
Position
droop
Time
D/A output rang
Time
Time
Time