Re: gearmotors,encoders

--- In CAD_CAM_EDM_DRO@egroups.com, Jon Elson <jmelson@a...> wrote:
<<SNIP>>
> Oh, very GOOD! Thanks for the measurement! Well, this is REAL
> bad news for the far end encoder crowd! Now, I can say with
authority,
> DO NOT DO THIS! IT will DEFINITELY not work with modern servo
> amps. I have rolloff starting at 1 KHz on my servo amps, with some
> power gain up to about 6 Khz. So, this thing would ring like a bell
> on my servo amps, and any others with high bandwidth. My ancient
> Allen-Bradley 7320 CNC control had a positioning loop bandwidth
> of only 2 Hz, and presumably was designed to connect to servo
> amps with low bandwidth also, perhaps 100 Hz or so. This is fine
> for a 20-ton bed mill, but totally inadequate for a Bridgeport or
lighter
> machine. Cutting forces can deflect the machine way away from the
> desired position before such an amp could catch up!
>
> Any reasonable servo system for a Bridgeport or lighter machine
should
> have at least several hundred Hz bandwidth on the velocity loop, and
> that puts you right into the area where the leadscrew could ring in
the
> torsional mode!
>
> Actually, the 180 Hz seemed kind of lower than I would have
expected,
> depending on the moment of that wrench. But, it clearly indicates
the
> far end approach is bound for trouble!
>
> The P&W machine that someone mentioned used that design was
> probably used with a low (or no) bandwidth servo system.
> Didn't the original "tape-o-Matic" use a pneumatic control?
> Or was that the original "Cint-i-Matic"?
>
> Jon

That was me with the Tape-O-matic. Last winter, when I got the
beast, the group looked at some of the hard to read schematics that
are on the RCM file site, and indicated that the amp bandwidth was
low. (I want to say even less than the 100Hz mentioned above, is
10Hz possible? I didn't go back in the archives and search for the
reply.) I'm glad I now asked he question!! It seems likely that
I'll have to remount the encoders when I conduct the brain transplant
on the P&W.
I'm still not quite sure how the possibility of this twist is
worse than the possible lash in the old cogged belts that connect the
servo moto to the leadscrew. Especially if either cog is bored
eccentric. Won't this be a greater risk than the screw twis &
resonance?
The tape-O-matic is a bit larger than a Bridgeport, maybe twice its
size.