Siemens SINUMERIK 802D sl Function Manual page 67

Operating principle
LookAhead functionality is available only for path axes, but not for the spindle.
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 sawtooth-shaped
velocity profile which can be avoided by reducing the set velocity for the number of blocks
considered by the LookAhead function (fixed value).
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.
Selection and deselection of LookAhead
If the continuous-path mode (G64) is selected LookAhead is called and de-
selected/interrupted with G60/G9.
Turning, Milling, Nibbling
Function Manual, 11/2012, 6FC5397-1CP10-5BA0
Continuous Path Mode, Exact Stop and LookAhead (B1)
3.5 LookAhead
67