Re: gearmotors,encoders

Thanks, Mariss. I understand what you're saying about no-windup
connections between the motor and screw, and I suppose that also
includes gearmotors.

What do you think about part 2), putting the encoder on the non-motor
end of the screw? Is there likely to be a problem due to, ah what
would an ME call it--the torsional flexure of the lead screw? I'm
guessing it's going to be a lot easier to find surplus servos with
tachs than with 1000 line encoders.

Incidently at least one encoder mfr (Dynapar) suggests the fiendishly
expensive helical connectors for connecting encoders.

--Jack





--- In CAD_CAM_EDM_DRO@egroups.com, "Mariss Freimanis"
<geckohall@h...> wrote:

> Hi,
>
> The problem is backlash. The servomotor will thrash back and forth
> over a distance equal to the backlash, building up a good head of
> steam on each transit. Expect considerable vibration as it does so.
>
> The motor gets its guidance from the encoder. In the backlash

region,

> the motor can move, (and it will) without any compensating motion

of

> the encoder. The motor then is uncontrolled and will accelerate. If
> the backlash is large, the motor speed will be cosiderable when the
> encoder finally moves. It will then reverse direction and the

process

> will repeat.
>
> Mariss
>
> --- In CAD_CAM_EDM_DRO@egroups.com, jmw@c... wrote:
> >
> > ------------------------------------------------------------------

--

> -
> >
> > Apologies in advance for burning bandwidth; I've asked about this
> > before--but I still don't get it. Why not put rotary encoder(s)

on

> > the non-motor end of the screw rather than on the motor shaft? It
> > seems like one wants to know the position, direction and speed of
> the
> > screw rather than of the motor per se. So if it's no worse,
> possibly
> > even better, to have the encoder on the far end--then what would

be

> > wrong with a slightly flexible coupling (e.g. Lovejoy) between

the

> > motor and the screw? I can see that during accel and decel the
> motor
> > and screw could get out of phase, and maybe in some bitter state

of

> > the world this could feed on itself, but couldn't this problem be
> > tweaked away with a little gain correction? I like slightly
> flexible
> > connections; they absorb vibration, correct for slight
> misalignment,
> > would put some allowance for a few thou of axial heat-growth of

the

> > screw in the system, etc.
> >
> > Thanks.
> >
> > --Jack