Newsgroups: comp.robotics
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From: nagle@netcom.com (John Nagle)
Subject: Re: A sense of balance
Message-ID: <1993Feb1.044158.23790@netcom.com>
Organization: Netcom - Online Communication Services  (408 241-9760 guest) 
References: <1993Jan31.192458.24679@netcom.com> <1993Jan31.211938.9674@sbcs.sunysb.edu>
Date: Mon, 1 Feb 1993 04:41:58 GMT
Lines: 43

shane@cs.sunysb.edu (Shane Bouslough) writes:

>John Nagle (nagle@netcom.com) wrote:
>True.
>: The best approach is a sealed cup of liquid with four capacitive sensing
>: plates.  This gets you two axes of balance.
>Sounds good so far...
>: The choice of the liquid is tricky (you want a medium poise, no foaming,
>: and uniform viscosity over the operating temperature range) but worth
>: experimenting with.
>Hmmm... getting more complicated...
>: Etak, the vehicle navigation people, used to make these, packaged with a
>: rate gyro, but I don't know if they still do.  They did considerable work
>: to get the geometry and liquid characteristics right so that driving on
>: rough roads wouldn't introduce excessive noise.  
>Oops, I think you just blew my whole $$$ budget. This sounds a bit like
>overkill for a little six-legged thing. 

      This is one of those things that's really cheap to manufacture
but expensive to engineer.  I'm looking at one now.  The level sensor
is a circular plastic cup about 3/4" in diameter and 3/4" long, with four
metal plates attached to the outside of the cup by a cap on one end and
a cable tie on the other.  It's mounted directly to a PC board with the
support electronics.  The rate gyro is in the same package; that's a
motor with a disk of sheet metal on the shaft, mounted a short distance
from a PC board with capacitive sensing plates.  Three ICs (one big 
custom surface-mount), and about 30 discrete components.  Runs off +5,
produces analog outputs for both level and rate.  The whole thing is
about 3" x 3" x 5", and could be shrunk if necessary.  ETAK part number
02-1226, circa 1988.  ETAK charged me about $150, but it was a favor
from the company founder that they sold it; they aren't usually a 
component supplier.  Nevertheless, researchers might find it worthwhile
to contact them.  ETAK, Inc., Menlo Park, CA.

      A 2D tilt-only sensor could probably be built for under $25 if you
built a few of them.  They're small and simple.  For electronics, find
a "build a capacitance meter" article and go from there.  Try some
working fluids like automatic transmission fluid, auto coolant, or light
machine oil.  Use some sealable plastic container for the fluid.  Maybe
glue a smaller cylinder inside, so the liquid only flows between the
inner and outer cylinders, to reduce sloshing.  

					John Nagle
