90
D
Position Loop Gain
The responsiveness of the servo system is determined by the position loop gain. When a servo system
has high position loop gain, the responsiveness is greater and positioning can be faster. In order for
position loop gain to be raised, the mechanical rigidity and the characteristic frequency must be in-
creased. For general NC machine tools, the range is 50 to 70 (1/s); for general machinery and assembly
devices, it is 30 to 50 (1/s); for industrial robots, it is 10 to 30 (1/s). The factory setting for position loop
gain is 40 (1/s), so it should be lowered for systems with low rigidity.
Position loop gain is generally expressed as follows:
Position loop gain (Kp) =
Instruction command frequency (pulses/s)
Deviation counter’s residual pulse amount (pulses)
(1/s)
In addition, the system response setting is 1/Kp, so in order to increase the responsiveness, it is neces-
sary to increase the position loop gain. If a system has low rigidity or low characteristic frequency, in-
creasing the position loop gain sympathetic vibration of machinery will occur and an alarm will be gener-
ated.
If position loop gain is low, positioning time can be improved by using feed-forward control.
In addition to this method, positioning completion can be speeded up by using the bypass function.
Motor speed
High position loop gain
Low position loop gain
Time
PRM
No.
Parameter name
Factory
setting
Unit
Setting
range
Explanation
Cn-1b
Positioning comple-
tion range
3
Com-
mand
units
0 to 250
Sets the range for the positioning comple-
tion signal output. (Generally set according
to the precision required by the system.)
Increasing the positioning completion range
too much can cause the positioning
completion output to turn ON during low-
speed operation or other times when there
are few residual pulses.
Cn-1C
Bias rotational
speed
0
r/min
0 to 450
Used to reduce positioning time.
Positioning time will be decrease as the
bias rotational speed is increased, but will
become unstable if it is increased too
much. Increase the value gradually from 0
and observe the affect on the system.
Cn-1d
Feed-forward
amount
0
%
0 to 100
Position control feed-forward compensa-
tion.
D
Feed-forward Amount
The feed-forward amount is effective when the position loop gain is set to less than 25 l/s. It will not be
very effective when the position loop gain is higher than 25 l/s.
Operation
Chapter 3
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