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5 Servo Adjustment
MITSUBISHI CNC
5.4.3 Dual Feedback Control
If the motor and machine coupling or machine system's rigidity is low (ex. large machine, etc.) when using a closed loop
system, the response during acceleration/deceleration will vibrate and cause overshooting. This can cause the position
loop gain from increasing. The dual feedback function is effective in this case.
To validate the dual feedback function, use position feedback with a motor side encoder in ranges with high acceleration
to enable stable control. In ranges with low acceleration, use position feedback with the machine side encoder (scale).
This will make it possible to increase the position loop gain.
The state will approach the semi-closed loop system as the primary delay filter's time constant increases, so the position
loop gain limit will increase. Note that the limit of the position loop gain increased with the dual feedback function is the
same as the position loop gain limit for a semi-closed system that does not use a machine side encoder (scale, etc.). In
addition, the positioning time will increase as the primary delay filter time constant increases.
POINT
1. Dual feedback control is a function that compensates symptoms resulting from insufficient machine rigidity.
If there are items that can be improved on the machine (improvement of scale installation position, etc.)
improve those first.
2. The position loop gain limit will not increase compared to the semi-closed loop system even when using dual
feedback control.
ENC
SV051
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Position control
Primary
delay filter
High frequency
FB element
Position
command
Position droop
Servo
motor
Linear scale
Table
Speed
command
Position FB
Position FB
Dual feedback control
Low
frequency FB
element