Reference Manual
Turbo PMAC/PMAC2 Software Reference
Turbo PMAC Global I-Variables 180
Isx20 Coordinate System x Lookahead Length
Range: 0 – 65,535
Units: Isx13 segmentation periods
Default: 0
Isx20 controls the enabling of the lookahead buffering function for Coordinate System x, and if enabled,
determines how far ahead the buffer will look ahead.
If Isx20 is set to 0 (the default), the buffered lookahead function is not used, even if a lookahead buffer
has been defined.
If Isx20 is set to 1, points are stored in the lookahead buffer as they are calculated, but no lookahead
velocity or acceleration-limiting calculations are done. The stored points can then be used to back up
along the path as necessary.
If Isx20 is set to a value greater than 1, PMAC will look Isx20 segments ahead on LINEAR and CIRCLE
mode moves, provided that the coordinate system is in segmentation mode (Isx13 > 0) and a lookahead
buffer has been defined. The lookahead algorithm can extend the time for each segment in the buffer as
needed to keep velocities under the Ixx16 limits and the accelerations under the Ixx17 limits.
For proper lookahead control, Isx20 must be set to a value large enough so that PMAC looks ahead far
enough that it can create a controlled stop from the maximum speed within the acceleration limit. This
required stopping time for a motor could be expressed as:
17Ixx
16Ixx
max
A
max
V
StopTime
All motors in the coordinate system should be evaluated to see which motor has the longest stopping
time. This motor’s stopping time will be used to compute Isx20.
The average speed during this stopping time is V
max
/2, so as the moves enter the lookahead algorithm at
V
max
(the worst case), the required time to look ahead is StopTime/2. Therefore, the required number of
segments always corrected in the lookahead buffer can be expressed as:
13Isx*17Ixx*2
16Ixx
)segsec/m(SegTime
2/sec)m(StopTime
eadSegmentsAh
Because Turbo PMAC does not completely correct the lookahead buffer as each segment is added, the
lookahead distance specified by Isx20 must be slightly larger than this. The formula for the minimum
value of Isx20 that guarantees sufficient lookahead for the stopping distance is:
eadSegmentsAh*
3
4
20Isx
If a fractional value results, round up to the next integer. A value of Isx20 less than this amount will not
result in velocity or acceleration limits being violated; however, the algorithm will not permit maximum
velocity to be reached, even if programmed.
Isx20 should not be set greater than the number of segments reserved in the DEFINE LOOKAHEAD
command. If the lookahead algorithm runs out of buffer space, Turbo PMAC will automatically reduce
Isx20 to reflect the amount of space that is available
Example:
The axes in a system have a maximum speed of 24,000 mm/min, or 400 mm/sec (900 in/min or 15
in/sec). They have a maximum acceleration of 0.1g or 1000 mm/sec
2
(40 in/sec
2
), and a count resolution
of 1m. A maximum block rate of 200 blocks/sec is desired, so Isx13 is set to 5 msec. The parameters
can be computed as:
Ixx16 = 400 mm/sec * 0.001 sec/msec * 1000 cts/mm = 400 cts/msec
Ixx17 = 1000 mm/sec
2
* 0.001
2
sec
2
/msec
2
* 1000 cts/mm = 1.0 cts/msec
2