Reference Manual

Turbo PMAC/PMAC2 Software Reference
Turbo PMAC Global I-Variables 182
is used to compute the starting axis positions on an R (run), S (step), or PMATCH command. The inverse-
kinematic program is used to convert programmed axis positions to motor positions each programmed
move or move segment for each motor defined as an inverse-kinematic axis (#xx->I). Motors in the
coordinate system not defined as inverse-kinematic axes still use axis-definition equations to convert
programmed axis positions to motor positions.
Isx53 Coordinate System x Step Mode Control
Range: 0 - 1
Units: none
Default: 0
Isx53 controls the action of a Step s command in Coordinate System x. At the default Isx53 value of 0, a
Step command causes program execution through the next move, DELAY, or DWELL command in the
motion program, even if this takes multiple program lines.
When Isx53 is set to 1, a Step command causes execution of only a single program line, even if there is
no move, DELAY, or DWELL command on that line. If there is more than one DWELL or DELAY
command, a single Step command will only execute one of the DWELL or DELAY commands.
Regardless of the setting of Isx53, if program execution on a Step command encounters a BLOCKSTART
statement in the program, execution will continue until a BLOCKSTOP statement is encountered.
On non-Turbo PMACs, this function is controlled by global I-variable I53.
Isx78 Coordinate System ‘x’ Maximum Circle Acceleration
Range: Non-negative floating-point
Units: (user position units) / (Isx90 feedrate time units)
2
Default: 0 (disabled)
Isx78, if set to a positive value, specifies the maximum centripetal acceleration that Turbo PMAC will
permit for a feedrate-specified (F) CIRCLE-mode move in Coordinate System ‘x’. If the move as
programmed would yield a higher centripetal acceleration, Turbo PMAC will automatically lower the
programmed speed for the move so that the Isx78 limit is not exceeded. The centripetal acceleration is
expressed as:
R
V
A
2

This limitation is done at the initial move calculation time, so it is not required to use the special
lookahead buffer in conjunction with Isx78. It still may be desirable to use the special lookahead buffer,
especially to manage the tangential acceleration into and out of a reduced-speed arc move.
The most important difference between limiting centripetal acceleration with Isx78 and limiting it with
the individual motor Ixx17 acceleration limits has to do with the exact path generated. Turbo PMAC’s
circular interpolation algorithms introduce a radial error term that can be described by:
R
TV
E
6
22

where V is the speed along the arc determined by the motion program (possibly limited by Isx78), T is the
Isx13 segmentation time, and R is the radius of the arc. If Isx78 is used to limit the speed of the arc, this
error will be reduced. However, if the special lookahead is used to limit the speed, the error will be as
large as if the arc move were run at full speed.