Siemens SINUMERIK 840Di Function Manual page 187

Override points
If the velocity profile of the following block velocity is not sufficient because, for example,
very high override values, e.g., 200%, or constant cutting rate G96/G961 are being used,
with the result that the velocity must be further reduced in the following block, LookAhead
provides a way of reducing the programmed velocity over several NC blocks.
By defining override points, LookAhead then calculates a limiting velocity profile for each
value. The required velocity reductions for the current override are derived from these
profiles.
The calculated maximum value of the velocity profile is limited by the maximum path velocity.
The higher value must take account of the velocity range reached by the maximum value of
machine data:
MD12030 OVR_FACTOR_FEEDRATE
(analysis of path feedrate override switch)
or the value of machine data:
MD12100 OVR_FACTOR_LIMIT_BIN
(limit of binary-coded override switch)
respectively.
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 for each channel is specified in machine data:
MD20430 LOOKAH_NUM_OVR_POINTS
(number of override switch points for LookAhead).
The corresponding points are written to machine data:
MD20440 LOOKAH_OVR_POINTS
(override switch points for 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.
Continuous-Path Mode, Exact Stop, LookAhead (B1)
Function Manual, 08/2005 Edition, 6FC5397-0BP10-0BA0
Detailed Description
2.4 LookAhead
2-39