Reference Manual

Turbo PMAC/PMAC2 Software Reference
Turbo PMAC Global I-Variables 99
programmed moves, the Ixx17 limit in this mode cannot guarantee that the limits will be observed.
Special lookahead is required for this capability.
In this mode, the Ixx17 acceleration limit can lower the speed of short programmed moves, even if they
are intended to be blended together at high speed. The algorithm limits the speed of each move so that it
can decelerate to a stop within that move. Without special lookahead, it cannot assume that it will blend
at full speed into another move.
2. Segmented LINEAR and CIRCLE mode moves with lookahead: If the Isx13 segmentation time
parameter for the coordinate system containing Motor xx is set greater than 0, put the coordinate system
in segmentation mode and the special multi-block lookahead function is active (lookahead buffer defined
and Isx20 greater than 0). Then Ixx17 serves as the maximum acceleration for Motor xx in all segments
of LINEAR and CIRCLE mode moves in the coordinate system. If a segment of one of these
programmed moves requests a higher acceleration magnitude of this motor, the segment time for all
motors in the coordinate system is extended so that the motor will not exceed this parameter, yet full
coordination is maintained. Furthermore, the Turbo PMAC will work back through already calculated,
but not yet executed, segments, to make sure the change in this segment does not cause violations in any
of those segments.
Note:
Ixx17 is not used for segmented LINEAR and CIRCLE mode moves when the
special lookahead buffer is not active.
The Ixx17 acceleration limit calculations assume that the coordinate system is operating at the %100
override value (real-time). The true acceleration will vary proportionately with the square of the override
value.
The use of the Ixx17 limit permits the setting of very small TA and/or TS values (Ixx87 and Ixx88 by
default). Do not set both of these values to 0, or a division-by-zero calculation error could occur. It is
advised that the TA time is set no smaller the minimum programmed move block time that should occur.
Example:
Given axis definitions of #1->10000X, #2->10000Y, Isx13=0 and Ixx17 for each motor of 0.25, and
the following motion program segment:
INC F10 TA200 TS0
X20
Y20
The rate of acceleration from the program at the corner for motor #2 (X) is ((0-10)units/sec * 10000
cts/unit * sec/1000msec) / 200 msec = -0.5 cts/msec
2
. The acceleration of motor #2 (Y) is +0.5 cts/msec
2
.
Since this is twice the limit, the acceleration will be slowed so that it takes 400 msec.
With the same setup parameters and the following program segment:
INC F10 TA200 TS0
X20 Y20
X-20 Y20
The rate of acceleration from the program at the corner for motor #1 (X) is ((-7.07-7.07)units/sec * 10000
cts/unit * sec/1000msec) / 200 msec = -0.707 cts/msec
2
. The acceleration of motor #2 (Y) is 0.0. Since
motor #1 exceeds its limit, the acceleration time will be lengthened to 200 * 0.707/0.25 = 707 msec.
Note:
In the second case, the acceleration time is made longer (the corner is made larger)
for what is an identically shaped corner (90
o
). In a contouring XY application, this
parameter should not be relied upon to produce consistently sized corners without
the special lookahead algorithm.