Karl Townsend wrote:
I could never get it to run at super slow speed in voltage mode, but
could easily do so in tach mode. It was kind of nice to see.


Disclaimer:My control uses a Galil dedicted real time controller so your
results may differ.

Torque mode is the preferred mode for most all applications, certainly for
what you're doing. Precise smooth control is not an issue at all with lathe
and mill applications.

Why do you say this? Most of the older machines had tach feedback and
velocity servo
amps, and many still do, in one form or another. Are you serious that
"precise smooth control"
is not an issue in machining applications? Or, are you saying that
torque mode control can be just as
smooth?

The problem with no-tach servo control is that at SOME point, as you go
slower, the encoder
counts come in farther and farther apart in time, and the CNC control
has no velocity
information between encoder counts. This is why some CNC controls
(Fanuc, for example)
moved to ever higher and higher encoder counts, now Fanuc is at 4
million counts/rev.
Try pricing a million cycle/rev encoder!

When I first set up my Bridgeport, I wanted to see how well the velocity
servo loop worked,
so I moved at ever slower rates until the movement started to become
irregular. That was at
about 0.01 IPM, or 3 encoder counts/second on this setup. With the EMC2
servo loop
operating at 1000 cycles/second, that meant it was going 333 updates
before seeing each count.
But, the velocity servo was keeping the movement quite steady.


Jon