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Installation manual
CNC 8055
CNC 8055i
7.
CONCEPTS
SOFT: V02.2X
Feedback combination for SERCOS axes with external feedback to
the CNC
7.20 Feedback combination for SERCOS axes with external feedback to
the CNC
On large machines that have a lot of backlash and use external feedback to obtain greater accuracy
may suffer some instability. This kind of machines run smoothly with internal feedback, but it may
lose precision; with external feedback, however, the accuracy improves but the machine movements
may be jerky. Combining both feedbacks both precision and smoothness may be achieved.
The feedback combination is handled with axis machine parameter FBMIXTIM (P102) and is valid
for linear and rotary axes.
The CNC uses feedback combination to calculate the feedrate. The CNC uses the external feedback
(direct feedback) to calculate the compensations, the circularity test, etc.
Feedback combination can only be used when the external feedback goes to the CNC. If the external
feedback goes to the drive, feedback combination will be disabled.
Cases where feedback combination is not supported:
• Spindles.
• CAN axes.
Feedback combination is compatible with the monitoring of the difference between the first and
second feedbacks. Even if monitoring is not active, when detecting that the accumulated feedback
difference between the first (internal) feedback and the second (external) feedback is too large, the
CNC shows an error message and opens the loop.
Use variables MIXPO(X..C) and FLWAC(C..C) to know the axis position with combined feedback
and the real error considering the second feedback.
In order for the feedback combination to work properly, the counting direction of the internal and
external feedbacks must be the same. If the counting direction of both feedbacks is not the same, it
must be changed using axis parameter AXISCHG (P13). Otherwise, the feedback combination will
not work.
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