Re: gearmotors,encoders

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

>
>
> Mariss Freimanis wrote:
>
> > Jack,
> >
> > Good morning! I see no problem mounting the encoder at the farr

end.

> > It's hard to imagine a leadscrew torsionally twisting even 1

encoder

> > count.
>
> Huh? It is most defintely possible. The longer and narrower the

leadscrew

> is, the worse the problem, of course. But, if you have screws 40"

long

> and under 1" diameter, the torsional deflection of the leadscrew
> can become quite significant with just tens of in-lbs of torque

applied.

> Take a long, narrow shaft, clamp one end, and see how little torque
> it takes to cause observable deflection with a wrench on the other

end.

> That will be far more than one encoder count!
>
> If this is a high power servo system, it would be a bad idea. For

a very

> low performance system, with low bandwidth, or if the screws are

fairly

> short and of large diameter, it will probably work OK.
>
> Jon

Hi,

Just did a little experiment, I like to have data backing up what I
say, otherwise my mouth (or fingers in this case) gets me into
trouble.

I took a 36" length of steel tubing (1" OD, 3/4" ID, uncertain
parentage regarding alloy) and clamped it in a vise at the far end. I
superglued a mirror sliver at the other end and trained a HeNe laser
on it. I put a torque wrench to the same end and cranked it until the
reflected laser beam on the wall indicated a torsional angle of 0.1
degree (equal to about 1 count on a 1,000 line encoder). The wrench
indicated 47 to 50 in/lbs torque, or about 800 in/oz.

I would assume a leadscrew of similar diameter would be a little
stiffer; better material and solid versus tubing.

I think there was a German tank in WWII that used torsion bar
suspension, but it weighed more than 50 tons. The moment arms seemed
quite long, so I expect the applied torque was considerably in excess
of what a garden variety servomotor can generate :)

Mariss

P.S. Resonant frequency of this mess (tubing + wrench) was 180 Hz. I
taped a differential large-area silicon cell on the wall where the
laser beam hit, AC coupled a 'scope to it to see the frequency.
Seemed kinda high.