Reference Manual

PMAC 2 Software Reference
164 PMAC I-Variable Specifiation
be expressed as:
13I*17Ix*2
16Ix
)segsec/m(SegTime
2/sec)m(StopTime
eadSegmentsAh ==
Because PMAC does not completely correct the lookahead buffer as each segment is
added, the lookahead distance specified by I1020 must be slightly larger than this. The
formula for the minimum value of I1020 that guarantees sufficient lookahead for the
stopping distance is:
eadSegmentsAh*
3
4
1020I =
If a fractional value results, round up to the next integer. A value of I1020 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.
I1020 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, PMAC will
automatically reduce I1020 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 1µm. A maximum block rate of 200 blocks/sec is
desired, so I13 is set to 5 msec. The parameters can be computed as:
• Ix16 = 400 mm/sec * 0.001 sec/msec * 1000 cts/mm = 400 cts/msec
• Ix17 = 1000 mm/sec
2
* 0.001
2
sec
2
/msec
2
* 1000 cts/mm = 1.0 cts/msec
2
• I1020 = [4/3] * [400 cts/msec / (2 * 1.0 cts/msec
2
*5 msec/seg)] = 54 segments