Sensors: "Where is the other end of my tether?"

There are several different approaches to determining a robot's
position "absolutely" (okay, relative to the Earth) such as GPS.
There are others for determining position relative to beacons and
walls where sound, light or RF are at least partially unobstructed.

But... suppose you want to build a tethered (remotely powered)
tunnel crawler to explore and map complex networks of metal piping,
such as sewers, conduits, and ventilation ducts? Odometry would be a
bit "sloppy" (especially in sewer pipes), and one couldn't depend on
light or sound.

Is there some simple method for determining where one end of a cable
is relative to the other end?  (Okay, how about a complex method?)
I know that if you throw enough money at the problem one can measure
the _length_ of the cable (via signal travel time), but what if you
want to know the far end's XYZ location relative to the beginning of
the cable after your robot has snaked its way through fifteen
air-duct conduit joints and created a replica of the Gordian knot?

If I build the cable out of "bend resistor" material I can
presumably determine the total amount of bending, but not what
directions the bends took.  Likewise (I think) with measuring the
refraction in a fiber optic cable.

With a crash (e.g.  20-year) research effort I suppose I could
create something out of microscopic flexible links with a built-in
pair of nano-potentiometers per joint that would tell me where every
bend was and the exact bend direction (within micro-pot tolerance
<grin>).  Trouble is, with the current state of the Frderal budget
getting research funds could be more difficult than it might
otherwise be.

Can anyone think of any other approaches? Even silly ones?


Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all)
--
   Totalitarianism is the crushing of ordinary, intractable, human
   reality  by a political idea.
                     -- David Horowitz / The Politics of Bad Faith
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