Siemens SINUMERIK 840D sl Function Manual page 202

B1: Continuous-path mode, Exact stop, Look Ahead
4.3 Continuous-path mode
Figure 4-6
Activation/deactivation
LookAhead is activated by selecting continuous-path mode G64, G641, G642, G643, G644
or G645.
Selecting the exact stop which works on a non-modal basis enables rounding to be interrupted
(G09).
LookAhead is deactivated by selecting the modal exact stop (G60).
Parameterization
Number of blocks
To achieve reliable axis traversal in continuous-path 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 default 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. It is sufficient 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 = 1 m/s
min, the following number of n
an attainable block cycle time of TB = 10 ms:
n
LookAhead
Considering these conditions, 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.
Since the machining velocity is very often set to a lower value than the maximum velocity in a
program, more blocks than are required would be predicted, overloading the processor
202
Example for modal velocity control (number of blocks considered by the LookAhead
function = 2)
LookAhead
= Deceleration path/Block length = ( v
and a high feedrate of v
2
blocks are allocated to the controller where it has has
/ (2a)) / (v
2
path path
Function Manual, 01/2015, 6FC5397-0BP40-5BA2
= 10 m/
path
* TB) = 9
Basic Functions