Alt-BEAM Archive

Message #04797



To: beam@sgiblab.sgi.com
From: Bruce Robinson Bruce_Robinson@bc.sympatico.ca
Date: Tue, 22 Jun 1999 18:35:04 -0700
Subject: [alt-beam] Re: Ah! Blessed Skepticism! (was: Feedback explained?)


James Wilson wrote:
>
> Here's my cents worth!
>
> When a motor is stalled (leg stuck) the inductor draws as much current
> as possible from the power source. The motor then happens to move if
> even a little bit and this will cause a back emf to increase the
> resistance of the circuit and thus influence the nervous net. Is this
> not apparent?

and John A. deVries II wrote:

> Ok, sure, people have -some- sort of qualitative observations indicating
> that a nervous net robot acts differently when a leg is stuck than when
> they the legs are free, but NO ONE seems to have made any repeatable
> experiments that either support or deny any particular hypothesis.

So let's see. Ian brought up the feedback idea. I refuted it. Ben
pointed out a flaw in my argument. I mentioned that there COULD be some
feedback at certain points in the walking cycle IF the supply voltage
was sucked way down. Steve pointed out that we were going over old turf.
And John said no one's actually done a proper experiment to
prove/disprove it.

After thinking about it some more, I realized that the main cause of
feedback to the Nv net would be when a motor was really stalled -- and
in that case the mechanical effects (motor barely moves) greatly
outweigh the Nv effects (Nv delay is shortened, reducing the amount the
motor will move).

And the previous discussions that Steve so kindly sent us (thanks,
Steve!) seem to support this point of view. When I look at the walker as
a complete system, I return to my original assertion, that inherent
feedback to the Nv is bogus. When (if) it IS present, the
electro-mechanical conditions that cause it have a far greater effect.

So to go back to John's statement, show me a repeatable experiment that
clearly demonstrates the effect, and I'll agree it's significant.
Otherwise I'm a confirmed skeptic.

And I promise not to say any more about this for a long, long time .

Take care,
Bruce

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