Subject:
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Re: why won't my biped walk?
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Newsgroups:
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lugnet.technic
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Date:
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Mon, 23 Aug 2004 00:25:35 GMT
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Viewed:
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3842 times
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In lugnet.technic, Kevin L. Clague wrote:
> OK, so I think I know why it won't walk.
>
> Right now, the rotation axis of the ankle is on the outer edge of the foot.
> This means that when the center post lifts the foot, the inside edge of the foot
> lifts, while the outside edge of the foot stays on the ground.
>
> Try rebuilding the foot with the two outer beams on the inside edge of the foot.
> I first noticed your very unusual configuration and didn't think much of it.
> Now that I think about it, your beam configuration is root cause.
>
> With the inside edge of the foot having the axis of rotation, when you lift the
> foot, you force the inside edge of the foot into the ground, and the outside
> edge of the opposite foot into the ground. This forces the COG to shift.
> Without both feet pushing the COG, you will not transfer from foot to foot.
>
> http://www.brickshelf.com/gallery/kclague/shorter-mm-walkers/pa230090.jpg
>
> Shows three minor variations of walkers, all which have the axis of rotation on
> the inside edge of the foot.
I think you're right. For the record, I tried simply shifting the foot plates
in one stud each -- and this does sort of work; the bot now wobbles a lot but,
most of the time, actually transfers from foot to foot.
But I think your analysis above is right. I had thought that the purpose of the
cam attached to the motor was primarily to lift the foot; thus, to get it to
also tilt the feet in the right direction, I had to attach it to the inside of
the foot rather than the outside.
But the result is essentially a race condition: on the one hand, we have the
lifting foot raising up (which tends to shift the COG toward that foot), and at
the same time, we are tilting the feet (which tends to shift the COG in the
other direction). With the feet coming in all the way to the center, the latter
effect can dominate -- but just barely. With them further apart, it can't, and
you get my first bot.
But after studying your reply above and the three bots in the picture, I think I
had it all wrong. The leg cams do not lift the foot; in fact, the "raised" foot
is the one attached to the cam in the up position. These walkers don't just
shift the COG for balance; it's actually the body tilt itself that raises the
foot.
I bet that's a lot more reliable, because not only do you have the pressure on
the inside of the foot as you describe above, but you also have the lifting foot
pressing down on the ground to cause the body to tilt over. I had surmised that
this would help, but hadn't taken it far enough -- I was still clinging to the
idea that I would actually need to lift that foot. :)
All right, time to tear my last biped apart and build a new one with this new
understanding! Thanks all -- you've been a big help.
Best,
- Joe
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Message has 1 Reply:
 | | Re: why won't my biped walk?
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| (...) Just a follow-up -- all I had to do was slightly redesign and reattach the feet, with the cam-driven connection on the outside and the stabilizing connections on the inside. It now walks quite reliably! Very cool. This bot is cute in a sort of (...) (20 years ago, 23-Aug-04, to lugnet.technic)
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Message is in Reply To:
| | Re: why won't my biped walk?
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| (...) OK, so I think I know why it won't walk. Right now, the rotation axis of the ankle is on the outer edge of the foot. This means that when the center post lifts the foot, the inside edge of the foot lifts, while the outside edge of the foot (...) (20 years ago, 22-Aug-04, to lugnet.technic)
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