Siemens SINUMERIK 840D sl Function Manual page 198

Detailed description
2.4 LookAhead
The number of blocks considered by the LookAhead function is limited by the possible
number of NC blocks in the IPO buffer.
Figure 2-13
Velocity profiles
In addition to the fixed, plannable velocity limitations, LookAhead can also take account of
the programmed velocity. This makes it possible to achieve a lower velocity by applying
LookAhead beyond the current block.
Following block velocity
One possible velocity profile contains the determination of the following block velocity. Using
information from the current and the following NC block, a velocity profile is calculated from
which, in turn, the required velocity reduction for the current override is derived.
The calculated maximum value of the velocity profile is limited by the maximum path velocity.
With this function it is possible to initiate a speed reduction in the current block taking
override into account such that the lower velocity of the following block can be achieved. If
the reduction in velocity takes longer than the travel time of the current block, the velocity is
further reduced in the following block. Velocity control is only ever considered for the
following block.
This function is activated by machine data:
MD20400 $MC_LOOKAH_USE_VELO_NEXT_BLOCK (LookAhead at programmed following
block velocity)
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.
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Example for modal velocity control (number of blocks considered by the LookAhead
function = 2)
Basic logic functions: Continuouspath Mode, Exact Stop, LookAhead (B1)
Function Manual, 11/2006, 6FC5397-0BP10-2BA0