PMAC Quick Reference Guide 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 t in msec vel Axis Letter t V P ⋅ = X5:50 V Distance P in user units, calculated from this page end velocity V in user_units per I190 msec I190 t Time vel vel vel V V V t V P ⋅ ⋅ = t V P ⋅ ⋅ = I190 3 t V 2 P ⋅ ⋅ ⋅ = I190 3 I190 2 t Time t Time t Time vel vel V V t V 2 P ⋅ ⋅ ⋅ = I190 3 t Time Motion Programs 57 Mode changer PVT 300 X 5 vel V V⋅t P = 2⋅I190 V⋅t P= 3⋅I190