Galil recently added support for Position-Velocity-Time (PVT) mode to their Accelera series of motion controllers. PVT mode is a common way of specifying motion trajectories. A trajectory is created from a series of motion segments, each with a specified move time, end velocity, and end positions. The controller then uses a third-order polynomial to smoothly interpolate the velocity and position between the defined points.
PVT mode provides much smoother motion than Galil’s old Contour Mode (also known as PT mode). In PT mode, trajectories are defined as a series of positions and times. The controller moves between the points at a constant velocity. Because each segment has its own velocity (the segment distance divided by the segment time), the controller has to ask for infinite acceleration between each segment. This leads to noisy motion, increased wear on components, and higher power consumption.
The two images below show the difference between PT and PVT modes. The image on the left shows PT mode. The quick changes in velocity can be seen at the transitions between trajectory segments. On the right, PVT mode is used to move through the same positions. As the image shows, PVT mode blends the velocity smoothly between motion segments, keeping the required acceleration as low as possible.
Galil added two commands to support PVT mode. The PV command is used to define the trajectory. Its format is PVn=p,v,t where n is the axis, p is the distance for the segment, v is the velocity for the end of the segment, and t is the time for the segment. The BT (Begin Trajectory) command operates just like standard BG command and is used to start motion on multiple axes at the same time.
For example:
PVA=5000,15000,512
PVA=5000,0,512
PVA=0,0,512
PVA=5000,0,200
PVA=0,0,0
PVB=5000,0,512
PVB=-5000,0,512
PVB=5000,0,512
PVB=0,0,200
PVB=0,0,0
BTAB
EN
You can read about this and other topics in the October issue of Galil’s ServoTrends newsletter.
