Reference Manual

Turbo PMAC/PMAC2 Software Reference
Turbo PMAC Global I-Variables 177
Isx13 Coordinate System x Segmentation Time
Range: 0 - 255
Units: msec
Default: 0
Isx13 controls whether Coordinate System x is in segmentation mode or not, and if it is, what the
segmentation time is in units of milliseconds.
If Isx13 is greater than zero, Coordinate System x is in segmentation mode, and all LINEAR and CIRCLE
mode trajectories are created by computing intermediate segment points with a coarse interpolation
algorithm every Isx13 milliseconds, then executing a fine interpolation using a cubic spline algorithm
every servo cycle.
While it is possible to execute programmed moves (blocks) of a shorter time than the Isx13 segmentation
time, the segmentation algorithm will automatically skip over these blocks, effectively performing a
smoothing function over multiple blocks.
This coarse/fine interpolation method activated by putting the coordinate system into segmentation mode
is required for the coordinate system to be able to use any of the following features:
 Circular interpolation
 Cutter radius compensation
 ‘/’ Program stop command
 ‘\’ Quick-stop command
 Rotary buffer blend-on-the-fly
 Multi-block lookahead
 Inverse kinematics
If none of these features is required in the coordinate system, usually it is best to leave Isx13 at the default
value of 0 to free up calculation time for other tasks. If Isx13 is 0, CIRCLE mode moves are executed as
LINEAR moves, cutter radius compensation is not performed, ‘/’ commands are executed as Q quit
commands, ‘\’ commands are executed as ‘H’ feed-hold commands.
Typical values of Isx13 for segmentation mode are 5 to 20 msec. The smaller the value, the tighter the fit
to the true curve, but the more computation is required for the moves, and the less is available for
background tasks. If Isx13 is set too low, Turbo PMAC will not be able to do all of its move calculations
in the time allotted, and it will stop the motion program with a run-time error.
The formula for the interpolation error introduced on a curved path by the segmentation mode is:
R6
2
T
2
V
E 
where V is the velocity, T is the segmentation time, and R is the local radius, all expressed in consistent
units. On a straight-line path, R is infinite, making the error equal to 0. If the velocity is expressed as a
feedrate F, in units per minute, the formula is:
 
 
 
 
R
10
1016.2
2
13Isx
2
F
unitsR6
2
secm
2
13Isx*
2
secm
2
000,60
2
min
*
2
min
2
units
2
F
E



















On non-Turbo PMACs, this function is controlled by global I-variable I13.
Example:
At a feedrate of 5000 mm/min (200 in/min), and a radius of 50 mm (2 in), a value of Isx13 of 10 msec
produces an interpolation error of 2.3 m (0.00009 in).