Siemens SINUMERIK 840D sl Function Manual page 201

Figure 4-5
Deceleration to velocity limits is possible with LookAhead such that violation of the acceleration
and velocity limit is prevented.
LookAhead takes plannable velocity limits into consideration such as:
● Exact stop at block end
● Velocity limit in the block
● Acceleration limit in the block
● Velocity limit on block transition
● Synchronization with block change at block transition.
Mode of operation
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.
Basic Functions
Function Manual, 01/2015, 6FC5397-0BP40-5BA2
Velocity control with short distances and exact stop G60 or continuous-path mode G64
with LookAhead
B1: Continuous-path mode, Exact stop, Look Ahead
4.3 Continuous-path mode
201