User's Manual

PMAC User Manual
180 Writing Programs for PMAC
PMAC Calculations
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.
Problems in Stepping
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 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.
Use to Create Arbitrary Profiles
The PVT mode is 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. PVT mode can
create any profile that any other move mode can. The following diagram shows common velocity
segment profiles.
Time
V
t
∆
P=1/2 Vt
Time
V
t
∆
P=1
/
3
V
t
Vel
Time
V
t
∆
P=2/3 Vt
V
el
Time
V
t
∆
P=
V
t
Vel
V
t
∆
P=1/2(V +V ) t
V
el
V
t
∆
P=5
/
6
V
t
1
2
V
/
2
∆
P=1
/
6
V
t
1
2
2t
2
V
1
Figure 33 PVT Mode Contouring (Hermite Spline)
Use in Contouring
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 the PMAC SPLINE mode, PVT produces a more accurate profile. Its worst-case error can
be estimated as: