Re: Encoders on the opposite end of the shaft

Jon, you keep giving me these doses of reality. Thanks, I needed
that.

If the encoder is best placed on the end of the motor, does that
pretty much quash the idea of using a cheap, single-end non-servo
PMDC motors? I suppose one could make another end bell for the back
of the motor, thread a hole axially in the back of the armature
shaft ... Sureley there's no problem putting a tach on the far end?

Thanks.

--- In CAD_CAM_EDM_DRO@egroups.com, Jon Elson <jmelson@a...> wrote:
>
>
> jmw@c... wrote:
>
> > Thanks, Brad, re the lore on your Tape 'O' Matic. If it's good
enough
> > for P&W it's good enough for me. Could you take a look at the
> > connection between the encoder and the screw? I assume the encoder
(s)
> > is rigidly mounted to the underside of the table and coupled to
the
> > screw with some sort of flexible coupling (what kind?)--or maybe
the
> > other way around--rigidly coupled to the screw and mounted to the
> > table on rubber. How did the P&W guys do it?
>
> Rubber is no good. It can develop a 'set' that can affect
accuracy. If
> the two shafts (leadscrew and encoder) are not very accurately
concentric,
> then a cyclical error will develop, even with most types of
couplings.
> The bellows coupling is the one most resistant to this.
>
> I used a one-piece helical-slitted coupling for my encoders. It is
working
> very well, and will probably outlast the rest of the machine.
>
> There are some other reasons to not have encoders on the opposite
end
> of the leadscrew from the motor. Torsional resonances can develop
at
> various frequencies, which change depending on where the nut is.
> That makes it hard to compensate the servo loop, so you end up
> reducing bandwidth substantially.
>
> Jon