Siemens SINUMERIK 840Di Function Manual page 188

Detailed Description
2.4 LookAhead
Fig. 2-14
... and the following settings:
MD20430 LOOKAH_NUM_OVR_POINTS = 2
MD20440 LOOKAH_OVR_POINTS = 1.5, 0.5
MD20400 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.
Machine data:
MD20450 LOOKAH_RELIEVE_BLOCK_CYCLE
(relief factor for the block cycle time)
can be set to smooth such velocity fluctuations.
Selection and deselection of LookAhead
The LookAhead function is selected and deselected with continuous-path mode (G64 or
G641 respectively).
2-40
Example for limiting velocity characteristics with number of LookAhead blocks = 4
Continuous-Path Mode, Exact Stop, LookAhead (B1)
Function Manual, 08/2005 Edition, 6FC5397-0BP10-0BA0