Siemens SINUMERIK 828D Function Manual page 173

For a machine with a low axial acceleration of a = 1 m/s
number of n blocks are allocated to the control where it has has an attainable block cycle time of TB =
LookAhead
10 ms:
n = Deceleration path/Block length = ( v
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 unnecessarily. For this reason, the
required number of blocks is derived from the velocity which is calculated from the following multiplication:
• Programmed velocity * MD12100 $MN_OVR_FACTOR_LIMIT_BIN
(when using a binary-coded feed-rate override switch)
• Programmed velocity * MD12030 $MN_OVR_FACTOR_FEEDRATE[30]
(when using a gray-coded feed-rate override switch)
The value for MD12100 or the 31st override value for MD12030 defines the dynamic response reserves which
the velocity control provides for when the path feed is overshot.
Note
The 31st override value for MD12030 should correspond to the highest override factor which
is actually used.
Note
The number of blocks considered by the LookAhead function is limited by the possible
number of NC blocks in the IPO buffer.
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.
• Determination of the 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.
The function is activated via the machine data:
MD20400 $MC_LOOKAH_USE_VELO_NEXT_BLOCK
Value Meaning
TRUE Function active
FALSE Function not active
Basic Functions
Function Manual, 09/2011, 6FC5397-0BP40-2BA0
B1: Continuous­path Mode, Exact Stop, LookAhead
2
and a high feedrate of v
2
/ (2a)) / (v * TB) = 9
path path
3.3 Continuous-path mode
= 10 m/min, the following
path
173