Alt-BEAM Archive
Message #04766
To: Beam mailing list beam@sgiblab.sgi.com,
From: Bruce Robinson Bruce_Robinson@bc.sympatico.ca
Date: Mon, 21 Jun 1999 23:46:34 -0700
Subject: [alt-beam] Heads: reinventing the wheel.
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While sitting around waiting for walker parts to arrive, I decided to
play around with head circuits for the first time. I came up with what
seemed like a good idea: use some kind of pulse generator to drive two
Nv's in parallel. Use some kind of photodevice (photo-resistors in my
case) to change the delay. The outputs of the Nv's go to opposite inputs
of the head motor (or driver).
Theory of operation: when both photocells receive the same amount of
light, the motor receives inputs on both sides, and so doesn't move. If
one photocell receives more light, its resistance is lower, so the Nv
it's attached to will time out first; since the othe Nv is still active,
this will turn the motor slightly. Made sense to me, so I tried it --
dismal failure. It worked reasonably well in dim light, but not in
bright light.
Now I save all the messages on BEAM circuits, even if I don't understand
them, so I went to my circuit folder to look up the recent discussion on
heads (which I didn't follow, but saved). And guess what I found? A
suggestion from Ben Hitchcock for pretty much the same circuit -- he
used photodiodes and an Nv oscillator, I used photocells and an
unbalanced bicore to create pulses. The principle was the same. I also
found a comment on this circuit from Wilf:
> ... The problem with this circuit is that the output pulse width becomes
> progressively narrower as the light level increases reducing output
> dutycycle to approach zero.
Yeah, well I'd just figured that out the hard way. But I'm a stubborn
cuss and it seemed to me there ought to be some way to make this work.
Besides, I didn't have the parts used in the other head circuits. I
thought about using the light-sensitive optics from modern welding
helmets to make a lens that would darken in bright light (too
expensive!), and I thought about some kind of primitive iris driven by a
tiny motor (too complicated).
And then I went back to basics. This circuit is like a race: two runners
going like mad to reach the finish line. But we're not interested in the
winner's time; all we're interested in is the time between when the
winner finishes and the loser crosses the finish line. We could care
less about the time while they're both running. So what we really want
to do here is once the winner crosses the finish line, get the loser to
slow down to a walk -- after all, he/she can't win, so why hurry.
In other words, what I wanted to do was have a "normal" delay through a
typical (1 meg, say) resistor, but use the photocell in parallel to
shorten the delay. But when one Nv timed out, I wanted to cut the other
photocell out of the circuit. After several complicated and unworkable
designs, I realized the answer was staring me in the face. Remarkably
simple, too.
Use a large resistor to ground as on a typical Nv. Hook up a photocell
in parallel, but instead of grounding it, run it to the output of the
other Nv, with an isolating diode. When the two Nv's are triggered, the
outputs both go low. This allows the photocells to charge the
capacitors quickly, until one of the Nv's times out, sending the output
high. That cuts the photocell to the still-active Nv out of the circuit
(thanks to the diode), allowing the active Nv to slow down and finish at
a leisurely pace.
And having learned the hard way, I tested this circuit before
broadcasting it, and it works very well indeed. You DO need to have well
matched photocells, however.
So, with all due credit to Ben, who thought of it first (even if I
didn't copy him) ...
Enjoy,
Bruce
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4767 Wed, 2 Jun 1999 09:37:02 +0100 [alt-beam] Re: teeny tiny robots "BEAM Mailing List" "Nicholas Smith" >got more inf fo on those bots , the y use muscle wire to expand and
contract
>kinda like a catipiller ,soooooooooooo anyone want me to post the pic? oh
>and when i said the cover had mark t's beam ant on it i meant andrew
millers
>"Big-Ant Bot", but they used mark's personal pcbs
>from '94 bye!
>
> john thurmon
YES INDEEDY! Post that picture! Unless anyone else on the list objects then
just send it to me!
Cheers
Nik
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