Karl Townsend wrote:

"Jon Elson" wrote in message
...


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?


Jon, I'm just saying the first rule of good engineering is KISS.

Torque mode is simple and will work very well for Iggy.

Depending on how smooth he wants it, and what his encoder resolution
works out to,
maybe yes, and maybe no. The whole basis of my argument for the tach
feedback is the
velocity servo is a continuous-time system, the DC tach and velocity
error amplifier are
not sampled in any way. (Yes, the PWM modulator, etc. downstream in the
typical servo
amp then makes it discontinuous-time, but the PWM frequency is high
enough that this
shouldn't interfere with my argument.)

Deriving velocity info from an encoder effectively samples the
information both in
position (encoder resolution) and in time (servo sampling rate), thus
introducing
a type of noise into the system, but also making the system run
open-loop between
encoder counts. At low speeds, this requires much lower gain of the
velocity loop, or
you get a step-wise response at each encoder count.

This is why Fanuc, for example, uses encoders with a million lines/rev.

Jon