Siemens SINUMERIK 840D sl Function Manual page 197

Scope
The LookAhead function is only available for path axes and not for spindles and positioning
axes.
LookAhead carries out a block-specific analysis of velocity limits and specifies the required
brake ramp profile based on this information. LookAhead is adapted automatically to block
length, braking capacity and permissible path velocity.
For safety reasons, the velocity at the end of the last prepared block must initially be
assumed to be zero because the next block might be very small or be an exact-stop block
and the axes must have been stopped by the end of the block.
With a series of blocks with high set velocity and very short paths, the speed can be
increased in each block depending on the velocity value currently calculated by the
LookAhead function in order to achieve the required set velocity. After this it can be reduced
so that the velocity at the end of the last block considered by the LookAhead function can be
zero.
This results in a serrated velocity profile (see the following fig.) which can be avoided by
reducing the set velocity or increasing the number of blocks considered by the LookAhead
function.
Number of blocks
To achieve reliable axis traversal in continuouspath mode, the feedrate must be adapted
over several blocks. The number of blocks considered by the LookAhead function is
calculated automatically and can, if required, be limited by a machine data. The standard
setting is 1, which means that LookAhead only considers the following block for velocity
control.
Because LookAhead is especially important for short blocks (relative to the deceleration
path), the number of blocks required is of interest for LookAhead braking (see fig. below). It
is enough to consider the path length to be equal to the deceleration path that is required to
brake from maximum velocity to standstill.
For a machine with a low axial acceleration of a = 1m/s
m/min, the following number of blocks for the control is obtained with an attainable block
cycle time of the control of TB = 10 ms.
n
LookAhead
Considering these aspects, it is advisable to adapt the feedrate over 10 blocks. The number
of blocks entered for the LookAhead function forecast does not change the LookAhead
algorithm and memory requirement.
As, in a program, the machining velocity is very often set to a lower value than the maximum
velocity, more blocks than are required would be predicted, overloading the processor
unnecessarily. For this reason, the required number of blocks is derived from the velocity
which is calculated from the programmed velocity multiplied by the value in the machine
data:
MD12100 $MN_OVR_FACTOR_LIMIT_BIN (limit for binary coded override switch)
Another possibility is multiplication with the 31st override value of the machine data:
MD12030 $MN_OVR_FACTOR_FEEDRATE (evaluation of the path feed rate override
switch)
The 31st override value must tally with the override factor most frequently used.
Basic logic functions: Continuouspath Mode, Exact Stop, LookAhead (B1)
Function Manual, 11/2006, 6FC5397-0BP10-2BA0
= Deceleration path/Block length = ( v
and a high feedrate of v
2
/ (2a)) / (v * TB) = 9
path 2
path
Detailed description
2.4 LookAhead
= 10
path
45