Siemens SINUMERIK 840D sl Function Manual page 199

The calculated maximum value of the velocity profile is limited by the maximum path velocity.
The upper point should cover the velocity range that will be reached by the maximum value
set in the machine data:
MD12030 $MN_OVR_FACTOR_FEEDRATE (evaluation of the path feed rate override
switch)
It can also be reached via the value of the machine data:
MD12100 $MN_OVR_FACTOR_LIMIT_BIN (limit for binary coded override switch)
In this way, a reduction of the velocity continuing into the block in which it is programmed
can be avoided. If velocity reductions across block boundaries are required already at 100%
override, a point must be set in the lower override range as well.
The number of override points used per channel is specified in the machine data:
MD20430 $MC_LOOKAH_NUM_OVR_POINTS (number of override switch points for
LookAhead)
The associated points are stored in the machine data:
MD20440 $MC_LOOKAH_OVR_POINTS (prepared override velocity characteristics with
LookAhead)
A combination of both procedures can be used to calculate the velocity profile and is
generally recommended, because the preset machine data for these functions already takes
the widest range of override-specific velocity limits into account.
Plannable velocity limits restrict override-specific velocity limits.
If neither of the two procedures has been activated, the setpoint velocity is always applied in
the current block.
Basic logic functions: Continuouspath Mode, Exact Stop, LookAhead (B1)
Function Manual, 11/2006, 6FC5397-0BP10-2BA0
Detailed description
2.4 LookAhead
47