Siemens SINUMERIK 840D sl Programming Manual page 332

Path action
11.2 Continuous-path mode (G64, G641, G642, G643, G644, G645, ADIS, ADISPOS)
Note
If FGROUP does not contain all the path axes, there is often a step change in the velocity at
block transitions for those axes excluded from FGROUP; the control limits this change in
velocity to the permissible values set in MD32300 $MA_MAX_AX_ACCEL and MD32310
$MA_MAX_ACCEL_OVL_FACTOR. This braking operation can be avoided through the
application of a rounding function, which "smoothes" the specific positional interrelationship
between the path axes.
LookAhead predictive velocity control
In continuous-path mode the control automatically determines the velocity control for several
NC blocks in advance. This enables acceleration and deceleration across multiple blocks with
almost tangential transitions.
Look Ahead is particularly suitable for the machining of movement sequences comprising
short traverse paths with high path feedrates.
The number of NC blocks included in the Look Ahead calculation can be defined in machine
data.
Continuous-path mode with smoothing as per distance criterion (G641)
With G641, the control inserts transition elements at contour transitions. The rounding
clearance ADIS (or ADISPOS for G0) specifies the maximum extent to which the corners can
be rounded. Within this rounding clearance, the control is free to ignore the path construct
and replace it with a dynamically optimized distance.
Disadvantage: Only one ADIS value is available for all axes.
The effect of G641 is similar to RNDM; however, it is not restricted to the axes of the working
plane.
Like G64, G641 works with LookAhead predictive velocity control. Corner rounding blocks
with a high degree of curvature are approached at reduced velocity.
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Programming Manual, 09/2011, 6FC5398-1BP40-2BA0
Fundamentals