Siemens SINUMERIK 840D sl Function Manual page 200

Detailed description
2.4 LookAhead
Figure 2-14
... and the following settings:
MD20430 $MC_LOOKAH_NUM_OVR_POINTS = 2
MD20440 $MC_LOOKAH_OVR_POINTS = 1.5, 0.5
MD20400 $MC_LOOKAH_USE_VELO_NEXT_BLOCK = 1
Block cycle problem
Block cycle problems are encountered in cases where the traversing distances of the NC
blocks to be processed are so short that the LookAhead function has to reduce the machine
velocity to provide enough time for block processing. In this situation, constant braking and
acceleration of path motion may occur.
Velocity fluctuations of this type can be dampened by the machine data:
MD20450 $MC_LOOKAH_RELIEVE_BLOCK_CYCLE (relieving factor for the block cycle
time)
Selection and deselection of LookAhead
The LookAhead function is selected and deselected with continuouspath mode (G64 or G641
respectively).
48
Example for limiting velocity characteristics with number of LookAhead blocks = 4
Basic logic functions: Continuouspath Mode, Exact Stop, LookAhead (B1)
Function Manual, 11/2006, 6FC5397-0BP10-2BA0