Alt-BEAM Archive

Message #05098



To: beam@sgiblab.sgi.com
From: Steven Bolt sbolt@xs4all.nl
Date: Thu, 8 Jul 1999 10:35:38 +0200 (CEST)
Subject: [alt-beam] Re: Something funny with the 1382 voltage triggers?


On Wed, 7 Jul 1999, Bob Shannon wrote:

> Suneaters simply have too many parts, and their power delivery to
> weight ratio is very poor when compared to the 1381 SE. The
> added complexity and weight rule out 'better' SE's in many
> applications where space and weight are critical.

Sure, Bob. SunEater_V - including two motors and solar panel -
weighs 14 grams, *much* too heavy, no doubt about it.
Btw, take care when visitors approach your RJP; they might
acidentally blow all your `bots away, if they are so much
lighter...

> The idea that the 1381 SE is not good 'in this application' is highly
> dependant on the specifics of the application.
---8<---

As you are finding out, and is causing you considerable
frustration, judging by your postings. The 1381 is difficult to
obtain but otherwise great. The 2-transistor latch/driver is not,
except when delivered in a package with the right motor and solar
panel. The motor must suck lots of current at half a volt, the
panel must supply very little current even in full sunlight. So
using a better motor or a larger panel gets you in trouble.
Even just about `correct' motors tend to help the latch/driver
oscillate, which it is all too happy doing (and obviously shouldn't).

The very large difference between the switch-on and switch-off
voltages - typically a factor four - just can't fit many
applications, as motors like their supply a bit more constant or
they won't be efficient. Even the rapid start/stop needs of most
photovores aren't served well by more than a factor two.
Of course there are exceptions, like a SolaRoller which should run
its entire race on one charge, and certainly shouldn't use its
motor as a brake. Even there you might want to do something
different, as 0.7V may be too *high* as switch-off level...

> Its not good engineering practice to make that choice without
> knowing all the design parameters.

So your design is working perfectly? Must have misread your postings.

> The 3904 is perfectly good in this application, provided that the
> start up current demand of the motor does not bring it out of
> saturation. Looking at the voltage drop from emitter to
> collector when the motors I'm using fire shows that a 2N2222
> would be overkill in this case.

When a motor needs so little current that a 3904 is the best
driver, it will often be too good to work well with the
2-transistor latch/driver. Of course such motors exist, but they
aren't common, and using a BC337-40, a 2N2219 or a 2N2222 will
*never* hurt. The BC337 comes in the same case as the 3904 (with
different pinout). Want something small and light? Then use for
instance the BC817-25. *Much* smaller case, be careful how you
breathe when working with those.

> (watch the on-line video clips of Vore-n-more, and pay close
> attention to its butt when the motors fire, notice how the energy
> is simply transfered between the suspension components, its not
> easy to see at that low frame rate, but its a very powerful
> effect. Without this effect, dynamic walkers are impossible.

I like your suspension idea. Very clever. But anything you do with
that 2-transistor latch/driver can be done better with other
circuitry, as you seem to be finding out, judging by your previous
postings. Earlier, you had much to say about advancing BEAM, if
memory serves; so why are you now so wedded to the first and worst
part of the BEAM circuit book? Very odd, methinks.

> The details of the application can become very important when you
> design for maximum practical efficiency rather than theorectical
> efficiency.

If there is a serious discrepancy, then your theory can't be much
good, can it? Storing energy in suspension springs is clever, but
surely not beyond theory.

> This we should leave as a matter of opinion. I think more people
> learn from the more popular SE's than they do from the 'better'
> ones.

I bow to your superior knowledge, and hope you enjoy that
`superior' circuitry...

> > Can you do better? Sure. Separate latch and driver. Then make sure
> > that your latch switches off properly (at well above 0.7V), or use a
> > monostable instead of a latch; that's more appropriate for most SE
> > applications.
>
> Ok, prove it. Show me the numbers.

It's a matter of reliably working with diverse motors and solar
panels, as your are finding out.

> How many centimeters per minute, under what lighting, and over what
> obstacles?

A SolaRoller race, or anything like that, will be decided by the
quality of the motor, the mechanical parts and the solar panel,
with the electronics coming in as a distant fifth.

> Or maybe my photovore can simply drag yours backwards faster than
> yours can go forwards?

Yeah, right. Photovore Sumo. Could be a good contest idea!
But the outcome is still decided mostly by the parts you didn't
design or make - motor and solar panel.

> Lets test this directly. I have little interest in arguing the
> point, but I am very keen to test it emperically, maybe we can
> all learn something new here.

Like what? I'm arguing in favour of properly designed, described
and explained circuitry, to make BEAM a better learning experience.
You're turning it into a "who can find or buy the best motors"
contest. Can be fun, but it's a different topic.

> Then tell us how bad the circuits really are. In some
> applications, doing work in the sun is more useful that just
> moving in dim light.

Of course it is. BEAM is about using sunlight, not darkness.
And your 2-transistor latch/driver-equipped `bot is having trouble
with sunlight rather than dim light, or have I missed something?

> I thought one of the beam ideals was the concept of a smart body?

Certainly, and you have trouble with the electronics right? Your
mechanical bits are doing fine, is what you're now rather
superfluously telling me.

> Perhaps it would be most efficient if I simply gave you a set of
> motors to play with?

Thanks. I'd like to take you up on that - though I'd reserve the
right to use my own motors - but my spare time is barely sufficient
for one contest. I'm very much enjoying the development of my
Easter Egg Hunter:

http://www.xs4all.nl/~sbolt/Other/e-EggHuntProgress.html

After that contest, I'll be happy to join your solar Sumo/tractor-pull.

Best,

Steve

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# sbolt@xs4all.nl # Steven Bolt # popular science monthly KIJK #
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