Reference Guide

PMAC Quick Reference Guide
Motion Programs 57
The units for position or distance are the user length or angle units for the axis, as set in the Axis
Definition statement. The units for velocity are defined as length units divided by time units, where the
length units are the same as those for position or distance, and the time units are defined by variable Ix90
for the coordinate system (feedrate time units). The velocity specified for an axis is a signed quantity.
From the specified parameters for the move piece, and the beginning position and velocity (from the end
of the previous piece), PMAC computes the only third-order position trajectory path to meet the
constraints. This results in linearly changing acceleration, a parabolic velocity profile, and a cubic
position profile for the piece.
Since a non-zero end velocity for the move can be specified (directly or indirectly), it is not a good idea to
step through a program of transition-point moves, and great care must be exercised in downloading these
moves in real time. With the use of the BLOCKSTART and BLOCKSTOP statements surrounding a series
of PVT moves, the last of which has a zero end velocity, it is possible to use a STEP command to execute
only part of a program.
The PVT mode is the most useful for creating arbitrary trajectory profiles. It provides a building block
approach to putting together parabolic velocity segments to create whatever overall profile is desired.
The following diagram shows common velocity segment profiles. PVT mode can create any profile that
any other move mode can.
PVT mode provides excellent contouring capability, because it takes the interpolated commanded path
exactly through the programmed points. It creates a path known as a Hermite Spline. LINEAR and
SPLINE modes are second and third order B-splines, respectively, which pass to the inside of
programmed points. Compared to PMAC’s SPLINE mode, PVT produces a more accurate profile.
Time
vel
V
t
I190
tV
P
⋅
=
PVT300
X5:50
end velocity V in
user_units per
I190 msec
Time t in msec
Distance P in user
units, calculated
from this page
Mode changer
Axis Letter
I1902
tV
P
⋅
⋅
=
vel
Time
vel
Time
vel
Time
V
t
V
t
I1903
tV
P
⋅
⋅
=
V
t
I1903
tV2
P
⋅
⋅⋅
=
vel
Time
vel
Time
vel
Time
V
t
I1902
tV
P
⋅
⋅
=
V
t
I1903
tV
P
⋅
⋅
=
V
t
I1903
tV2
P
⋅
⋅⋅
=