Alt-BEAM Archive
Message #05896
To: beam@sgiblab.sgi.com
From: Bruce Robinson Bruce_Robinson@bc.sympatico.ca
Date: Fri, 27 Aug 1999 12:55:33 -0700
Subject: [alt-beam] Re: Walker leg centralizing info
Jason - wrote:
>
> Hi all, recently the thing that made me dislike
> or bored of building bots is the fact that how
> to centralize a leg in a two motor walker....i
> heard stuff like mechanical stops and springs
> and etc. i am all tired of that and it can make
> the ot reall uncool and messy....
I'm surprised no one jumped on this one. So from me you get a long,
technical discussion.
The legs on a walker form part of a system. As they oscillate, you want
them to remain centred about one spot. So the goal of the system is to
keep moving the legs back toward the centre whenever they move away from
it. However, you don't want the system to respond TOO quickly, or the
legs will never move.
What this means in practical terms is this. You need something that will
measure where the legs are relative to the centre, and exert an
appropriate force to direct them back to the centre whenever they drift
away.
Springs do this very nicely when you arrange them proplerly. Electronics
can do it too, but you need to sense the position of the legs. I devised
a purely mechanical device that determines when the legs are moving too
far in one direction, and changes the bias on the microcore to prevent
it -- but if you though SPRINGS were ugly, you definitely will not like
this design (and it's very complicated).
The one thing that is very elegant is gravity. If you can arrange the
geometry of your motors and legs so that they will be centred when the
body is lowest (relative to the feet), then you may find your legs
staying centred. If you look carefully at some of Tilden's walkers,
you'll notice that the motors are often mounted so the shafts are NOT
perpendicular to the ground. This means that there will always be some
kind of restoring force caused by gravity.
The bottom line is, whatever method you use, there has to be some kind
of feedback that tries to restore the legs to centre: springs, gravity,
electronics. Gotta have something.
Regards,
Bruce
5897 Fri, 27 Aug 1999 20:45:02 -0400 [alt-beam] Re: New Guy Questions beam@sgiblab.sgi.com Bob Shannon Bruce Robinson wrote:
> Bob Shannon wrote:
> >
> > Yes actually.
> >
> > But I had just been reading Living Machines
> > again when the message came in. I started to
> > think, and realized that LEM does a similar
> > thing with AND gates between the Microcore
> > and Bicores, so I'm hoping that there's been
> > a change of heart about digital logic. I
> > think I'd overlooked that when I first read
> > the paper, so it kinds of took me by supprise.
>
> Digital logic? Change of heart? Bob, I don't see any taboo regarding
> digital logic in Living Machines. And I don't recall anyone badmouthing
> it on the BEAM list, either.
I'm not trying to start a debate, but in Living Machines does contain a few
statements such as:
"There is no notion of programming, but rather adaptive, parallel
reconfiguring of signals in neuron circuits, typically in ring topologies.
These structures compute, but not in any digital sense."
Later we read:
"In biomorphic architectures we do not have finite state machines in the
normal sense, scince there is no concept of digital or symbol, nor preset
update rules, only dynamic interactions among parts of the machine."
> Living machines makes reference to Nv and Nu neurons -- both of which
> use inverters -- which are digital chips. The common chips for bicores
> and microcores today are 74xx240's and 74HC14's, which are both digital
> chips. Lots of early (and present day) robots use 74xx245's, which are
> digital. Tilden's patent refers to the MC14584, which is a digital CMOS
> chip. And Tilden designed a "biomech bridge" for his robots which uses
> the 74xx139 -- another digital chip.
I think your confusing the original use of these chips, and their general
application
in BEAM circuits, where they are deliberately operated in an analog-like
way.
> Living Machines does make a comparison between "living machines" and
> microprocessor-based machines, and someone keeps mentioning a perceived
> anti-CPU bias on this list . But I don't see anyone claiming digital
> logic is a no-no.
>
> ... with a smile,
> Bruce
No matter, just checking.
But I had thought of a microcore as a state machine, an async. ring counter
actually.
And I hung conventional (digital) AND gates together to take the timing
pulses from
that ring counter and strobe my motor drivers.
This is not exactly whats described in Living Machines.
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