Re: re:cleaning, lovejoy, black box tach, linear scales, making pulleys

--- In CAD_CAM_EDM_DRO@egroups.com, Jon Elson <jmelson@a...> wrote:

>
> I find this VERY hard to believe. Maybe, with no realistic band-

limiting

> it is only 25 Db, but with reasonable band limiting (just a simple

RC

> at the input to the servo amp) I think I have demonstrated a SNR of
> over 40 Db on my own system. The tach is still working WELL down
> into the tens of uV region, but the finite gain of the amplifiers

is eventually

> overcome by stick-slip friction somewhere below .01 IPM. Maybe
> I'm just picking at rediculous straws, here, but I want it SMOOTH

down

> as far as I can possibly get it. (The idea is partly that if it is

smooth

> down to rediculously low velocities, it will also perform well

while holding

> position against intermittent external loads, ie. cutting forces.)
>
> Jon

Hi,

Jon, could you clear up something for me? Assuming a PID loop, it
seems there is no simple way to keep the motor from dithering or
bouncing between adjacent encoder edges due to the "infinite" DC gain
of the integral term. Does your system exhibt this phenomena, and if
it does, what is the frequency?

On an allied subject, quasi sine-cosine encoders are intriguing. If
you strip the slope information for sine -45 deg to sine +45, it is
pretty darn linear. Stitch these 90 degree segments together and you
get continuous position feedback.

I've got a very simple breadboarded circuit doing this right now with
a cheap ($100) DRC sine-cosine encoder I'm using in conjuction with a
hacked G320. The motor is completely still between steps because the
servo nulls at the sine=cosine points.

What is really exciting is when I replaced the STEP/DIRECTION
interface with a variable freq. constant amplitude sawtooth waveform.
The fast edge of the sawtooth is differentiated and forms the STEP
input; positive going is CW, negative going is CCW. This same
sawtooth is scaled and injected into the servo summing node.

The result is an absolutely smooth, monotonic motion down to zero
speed! On a much lesser note, the interface requires only a single
signal line. I am going to pursue this further to see if it is
practical.

Mariss
Geckodrive Inc.