Siemens SINUMERIK 828D Function Manual page 172

B1: Continuous­path Mode, Exact Stop, LookAhead
3.3 Continuous-path mode
Principle
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.
Figure 3-6 Example for modal velocity control (number of blocks considered by the LookAhead function = 2)
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 block-by-block basis enables rounding to be interrupted (G9).
LookAhead is deactivated by selecting the modal exact stop (G60).
Parameter assignment
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 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 enough to consider the path length to be equal to the
deceleration path that is required to brake from maximum velocity to standstill.
Basic Functions
172
Function Manual, 09/2011, 6FC5397-0BP40-2BA0