Siemens SINUMERIK 808D Function Manual page 38

Comparison of the G60 and G64 velocity behavior with short travels in the blocks:
LookAhead takes plannable velocity limits into consideration such as:
● Velocity limit in the block
● Acceleration limit in the block
● Velocity limit on block transition
● Synchronization with block change at block transition
● Exact stop at block end during termination
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.
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Function Manual
01/2017