Bill, Art;
I might be able to expand on this somewhat.
In Mach2 jargon we often speak of look-ahead as if it were synonymous
with constant velocity contouring. Mach2 examines the forward toolpath
so as to be able to run through data points without starting and
stopping at each one (if possible).
From a control designer's point of view, however, look-ahead is often
used for other things. One technique, called adaptive look-ahead, is
used to reduce following error in servo systems. The path is analyzed
for accelerations (changes in direction) as in the previous example.
The information is fed to the profiling algorithm through a method
called velocity feed-forward - basically, another term in the PID
equation. Acceleration feed-forward, if used, adds still another term.
These combine to give the servo advance warning that a corner is
coming, allowing it to set up for the corner as they say in dirt track
racing.
Adaptive look-ahead is more or less standard fare these days in high
speed machining. The benefits are of particular importance to laser
and plasma machines. It substantially improves the value of CV
contouring.
Unfortunately, adaptive look-ahead is not possible in Mach2. The
technique requires that the position loop be closed in the computer.
Mach2 leaves this to the drive. Rutex/Gecko servo drives perform the
PID based on immediate position data.
Doug
