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4. Servo Adjustment
4 - 33
4-4 Adjustment during full closed loop control
4-4-1 Outline
(1) Full closed loop control
The servo control is all closed loop control using the detector's feedback. "Full closed loop control"
is the system that directly detects the machine position using a linear scale, whereas the general
"semi-closed loop" is the one that detects the motor end position.
In a machine that drives a table with a ball screw, the following factors exist between the motor and
table end:
[1] Coupling or ball screw table bracket's backlash
[2] Ball screw pitch error
These can adversely affect the accuracy. If the table position, which is the tool end, is directly
detected with a linear scale, high-accuracy position control which is not affected by backlash or
pitch error is possible. However, with the full closed loop system, the machine system is also
directly included in the position loop control. Thus, if the machine's rigidity is not high, the gain
cannot be increased, and the required high accuracy cannot be attained.
The procedures for adjusting the servo with the full closed loop system are the same as the
semi-closed loop system. Vibration or overshooting will occur easily, so the position loop gain is
generally lower than the semi-closed loop.
(2) Overrun detection
With the full closed system, the tool end position feedback (FB) detected with the linear scale is
used for the position control. However, the motor end position FB is detected at the same time, and
the error of both FB is observed. If this FB error exceeds the servo parameter SV054 setting value,
alarm 43 will be detected and the system will stop to prevent overrunning due to a scale FB error
from occurring.
Overrun detection control
No. Abbrev. Parameter name Explanation Setting range
SV054 ORE Overrun detection
width in closed loop
control
Set the overrun detection width in the full-closed loop control.
If the gap between the motor end detector and the linear scale (tool end
detector) exceeds the value set by this parameter, it is judged to be
overrun and Alarm 43 will be detected.
When "-1" is set, the alarm detection won’t be performed. When "0" is
set, overrun is detected with a 2mm width.
-1 to 32767
(mm)
Position
command
PGN
+
-
FB error
Position droop
Servo-
motor
ENC
Linear scale
Table
Speed command
Machine end
p
osition FB
Motor end position FB
+
-