Alt-BEAM Archive
Message #04420
To: beam@sgiblab.sgi.com
From: Steven Bolt sbolt@xs4all.nl
Date: Sat, 12 Jun 1999 08:11:28 +0200 (CEST)
Subject: [alt-beam] Re: Turning a walker
On Sat, 12 Jun 1999, Ben Hitchcock wrote:
> Steven Bolt did this a while ago, using two motors, and EIGHT legs.
>
> I'm pretty sure you could make a frame with four legs that would
> work just as well, provided you used the same basic design.
That's not correct. Ken Huntington has tried with six legs and
appears to be doing well, but some `brain' control of the gait
becomes necessary, and manoeuvrability may suffer somewhat. The
wrong gait causes it to flip backwards rather easily. Four legs
would really not be enough.
With eight legs, the Spider doesn't need a brain to walk and
manages quite well in difficult terrain. Lifting its feet about
2cm, it walks all over the tools and keyboard on my desk, and
handles tallish gras by kind of swimming over it. There are two
recognizable gaits:
Big Step - 180 or 0 degree phase difference between any
left/right pair of legs
Wave - 90 degree phase difference between any
left/right pair of legs.
The `wave' looks most natural and avoids peaks in current
consumption; the average is a little lower than in `big step'.
The Spider brain will be taught to maintain `wave' when walking
straight.
The phase of legs on one side has to alternate (the first at 0, the
next at 180 degrees, then 0 again, followed by 180 for the fourth)
for walking to be good.
> The interesting thing about this design is that it is a walker,
> that doesn't need a microcore. All you need is one battery, two
> motors, and that's it electrically. I would hazard a guess that
> the gait would 'evolve' over a period of several seconds into
> something that approaches the alternating diagonal gait.
If left brainless, small differences between the motors will cause
all gaits to happen in repeating sequence.
> Deceptively simple, this one! You just treat the walker like you
> would treat a wheeled 'bot - to walk forward, you just run both
> motors forward. To reverse, you run both in reverse. To turn,
> you make one run, while the other either stops or goes in
> reverse.
In a way it is a wheeled `bot, with all `wheels' powered. Most
people don't consider this approach to be true walking, but it fits
my requirements:
1. Everything touching the ground has to be powered. Otherwise
mobility will be impaired in all but easiest environments.
2. More than two motors are out of the question. Adding motors
increases complexity and thus makes construction more difficult.
Cost also goes up fast, and not only because good motors are
expensive; you need more power to move the extra weight, and that
means larger batteries and/or solar cells.
3. The robot must be able to turn around its top axis, that is on
the spot. Otherwise horizontal mobility just won't be good
enough.
Best,
Steve
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# sbolt@xs4all.nl # Steven Bolt # popular science monthly KIJK #
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