Re: gearmotors,encoders

Mariss, wow, there's nothing like real data. Let's say a solid lead
screw 1.25" in diameter is five times stiffer than your test bar
(tube). That'd put things kind of on the boderline in terms of
loosing a count with a motor in the 250 in/oz range and operating at
capacity. I guess things are a little less favorable when the torque
is applied suddenly?

--Jack

<snip>

> 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.