Alt-BEAM Archive
Message #06809
To: beam@sgiblab.sgi.com
From: Jean auBois aubois@trail.com
Date: Sun, 17 Oct 1999 02:44:11 -0600
Subject: [alt-beam] Tilden used something that can be made into an Electronic
As part of the unexploded ordnance (UXO) project, Mark was using a product
that costs on the order of $30..40 (see way below for details) that was
rather interesting and quite appropriate to BEAM design. It is so
sensitive, however, that it couldn't be used while the robot actually was
in motion -- the magnetic fields of the motors screwed things
up. Additionally, I believe he had to mount it a piece of aluminum, but
that may have been in the interests of saving weight. The point of the
sensor, of course, was that the bombs and stuff were supposedly
sufficiently magnetic that you could detect them that way.
With regard to this alternation, on the Solarbotics site
(http://www.solarbotics.com),
Dave says:
>There is no waist motor, but it does have a functional light-seeking head.
>Stryder walks for approximately 30 seconds, stops, looks left and right
>for the best light source, and restarts walking, turning in the direction
>of the light.
without any particular explanation for the behavior -- after all, there is
no reason why Stryder has to stop at all. One might think it would be for
the reason Dave gives with regard to
>NeuSpotter, built in late 1997. This interesting device uses groups of
>PhotoPopper circuits to balance movement against head motions.
IMNSHO, however, the reason for the alternate sense-then-move style
behavior is based on the UXO robot. In other words, there are times that
it is valuable for your sensors to be stable (less jitter, for example) or
simply turned off (for example, if they'd be swamped by some robot-created
signal). Offhand, I think that the biological world tends to use this
scheme only when the creature isn't under any particular stress or is
stalking... more often, though, this is not at all a very biological
approach -- try walking towards some goal 30 feet away three steps at a
time with your eyes closed, open your eyes to determine where you are
going, repeat... and see what happens... try something similar with a glass
of water. The more biological approach is that sensing and movement occur
simultaneously.
About the sensor that Tilden used:
Well, son of a gun I actually went and searched my own stuff... The
manufacturer is Speake & Co., Ltd., the distributor is Fat Quarters
Software, the sensors are three wire devices that use 5v and the cheapest
(either the FGM-3 or the smaller FGM-1) is $37.50 -- see:
http://fqs.dconn.com/INDEX.asp
for their home page or
http://fqs.dconn.com/Electronics/sensors.htm
for the index to the individual sensors.
The output is a rectangular pulse whose -period- is directly proportional
to field strength (i.e. the frequency varies inversely with the
field.) Don Lancaster (Tech Musings) wrote an article about them in the
September 1996 issue of Electronics Now and there was an article in the
August 1996 Nuts & Volts magazine about them.
Other possible magnetic sensors:
A flux gate project:
http://shell3.ba.best.com/~beale/measure/fluxgate/index.html
Some interesting magnetic stuff (the magnet sliding slowly down the copper
tube idea is fascinating):
http://www.eskimo.com/~billb/neodemo.html
Stuff about Precision Navigation's stuff:
http://www.tri-m.com/products/precisionnav/sensors.html
One of their blurbs says:
>The sense coil serves as the inductive element in a very simple L/R
>relaxation oscillator, with its effective inductance being influenced by
>the ambient magnetic field component running parallel to the coil axis.
>The frequency of the square wave changes with applied magnetic fields, and
>the output of the Op-Amp can be fed directly into a microprocessor.
Incidentally, they sell a module that hooks onto a 3COM Palm-Pilot for
about $40 which implies that the basic "guts" has got to be pretty
cheap. A non-BEAM approach to the use of a different module they sell can
be found on:
http://www.phanderson.com/printer/compass/compass.html
Some other company altogether:
http://dinsmoregroup.com/dico/
They've got a "digital" device (8 values -- 45 degrees apart) for only $12
and analog units (sine-cosine outputs) for $35.
JaB
http://www.serve.com/heretics
An unconventional approach to the BEAM philosophy
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