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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Idea #1; The robot sets a small automatic theodite (survey head) at every other corner, and a target at the intermediary corners.
Idea #2; The robot sets off small explosions every foot as it travels, and you track the explosions by seismograph.
Idea #3; It asks for directions from roaches which it happens to meet along the way.
Joe Dunfee
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Joe,
Thanks for the suggestions. <grin>

Sounds good, as long as it picks up its toys when it's done. <grin>

Hey! I _like_ that one!

This will significantly reduce the mapping hardware, but I'll have to add in a sonic/odor-to-text translation module (at least, I assume that's how roaches communicate). Let's see: "A robot explores a sewer pipe where two tribes of roaches live. One tribe always tells the truth, and the other always lies. What one question can the robot ask the first roach it meets that will let it determine its absolute position?"
Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all)
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Interesting issue and interesting set of problems.
May be following links give you a clue or two; http://www.sewerobots.de/en/referenzen.html#sensor_data http://www.ais.fhg.de/projects/Makro/makro-engl/MAKRO-flyer-e.99.pdf http://www.nosc.mil/robots/pubs/icra2000b.pdf http://www.ais.fhg.de/~surmann/papers/sirs2001.pdf http://arxiv.org/ftp/cs/papers/0412/0412051.pdf http://csdl2.computer.org/persagen/DLAbsToc.jsp?resourcePath=/dl/proceedings/&toc=comp/proceedings/eurobot/1996/7695/00/7695toc.xml&DOI .1109/EURBOT.1996.551883 http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumberU4212
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Ertinko,
Thanks for the reply.

Nice list, but no direct links. However, the titles -- fed into Google -- did turn up some interesting results.

http://csdl2.computer.org/persagen/DLAbsToc.jsp?resourcePath=/dl/proceedings/&toc=comp/proceedings/eurobot/1996/7695/00/7695toc.xml&DOI .1109/EURBOT.1996.551883
I've downloaded some of these (IEEE tends to be a bit protective of its papers) and will look them over.
Thanks.
Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all)
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Check out Luna Innovations in Blacksburg VA. Their approach isn't simple, but it is impressive.
http://www.lunainnovations.com/Products/shape.asp
Brent S.
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Brent,
Aha! So it _can_ be done!
And those tantalizing phrases: "high-spatial resolution, distributed fiber-optic shape and position sensing cable ... this smart cable...'knows' its own position in 3D space at every discrete point along its length".
I'd take two if I knew what they both cost. <grin>
If it lives up to its promise... imagine dancers with this stuff wrapped around them to capture position data for virtual reality reproduction. Or to monitor athletes during training to provide feedback on what they're doing (vs. what they think they're doing).
Only trouble is that pesky "under development" part. Sigh.
Anyway, thanks for the pointer. Neat stuff!
Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all) -- The most exciting phrase to hear in science, the one that heralds new discoveries, is not Eureka! (I found it!) but "That's funny..." -- Isaac Asimov --
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"Frnak McKenney"

may be complex ($$$): Inertial Navigation?
HTH Martin
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Martin,
I really like the "smart cable" Brent found (see previous post), but it's off in the future. The inertial approach (measure where the end of the cable travels rather than try to obtain an after- the-fact position) is already "here" (and it's wireless -- the cable is invisible<grin>):
IMU 6 Degrees of Freedom - v2 with USB Bluetooth and Battery Holder http://www.sparkfun.com/shop/index.php?shop=1&cartB1120& cat=1&itemidI3&
Description: Complete wireless Inertia Measurement System - 6 degrees of freedom. Version 2 uses the single IC triple axis accelerometer from Freescale MMA7260Q and combines it with three iMEMs gyroscopes. Roll, Pitch, and Yaw gyro sensors including a total of 3 tilt axis. All measurements are transmitted via a wireless Class 1 Bluetooth link that is extremely easy to use with a range of 100m (330ft) line-of-sight and 30m (100ft) indoors.
Only minor flaw (for me) is the $350 price tag. TANSTAAFL.
Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all) -- Those who profess to favor freedom, and yet depreciate agitation, are men who want rain without thunder and lightning. -- Frederick Douglass, 1817-1895
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On Tue, 04 Oct 2005 14:12:31 GMT, Frnak McKenney

Presuming you're on a "tether" cable, you're probably unwinding it (either at the base end or on the robot, and from this (calculations from rotation count and winding diameter) you know the total distance of the tether you've unwound. With onboard gyroscopes, you can know how far you went in each direction as you unwound the cable. Calculation of current position is done with this info and trigonometry.

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On Tue, 04 Oct 2005 21:58:29 GMT, Ben Bradley

Ben,
Thanks for the reply.
A "Theseus thread" would certainly give the explorer a way home (or a way to attempt to haul it back or send another explorer after it).
Will the cable be taut enough to yield a good approximation of the distance travelled? I'm wondering about making multiple turns in large- diameter pipes, where the cable might "hike up" the wall and represent the szhortest path to the explorer, but not necessarily represent the distance actually travelled.
Of course, I may be overly optimistic on how much accuracy I can expect. <grin>
Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all) -- Some see private enterprise as a predatory target to be shot, others as a cow to be milked, but few are those who see it as a sturdy horse pulling the wagon. -- Winston Churchill, 1874-1965 --
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Here is a silly one - a little xmitter on the end of the cable, and use 3 (or more) receivers for triangulation - bit of work, but possible I should think with a software package to do the calcs
David
Frnak McKenney wrote:

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"quietguy"

Humnn, the OP said that intended environment is sewer pipes. How you're gonna make the signal travel through steel pipes? Not to say that they will probably be underground.
Now, someone else said something about little explosions. Following the same concept, if the environment is somewhat predictable (for example all pipes are steel or metalic), you could use sound to figure out the distance from base. It would be something like this:
Through your tether (I assume it has a data line also), you send a pulse and start a timer. When your robot receives that pulse, it actuates a device similar to a hammer, that will bang the wall of the pipe. In your base station, you will have a device that will listen to that bang and calculate the round trip time. Now, which direction and what turns it took I believe it's gonna be a bit more complicated.
Cheers
Padu
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Interesting. How about this:
- Put closely-spaced lights on the cable that flash in sequence. - The robot keeps the cable tight as the lights unwind. Perhaps it needs to install spring-loaded braces every now and then to accomplish this. - A video camera on the robot can observe the lights it leaves behind. - By tracking the amount of unwound cable, the robot can determine its distance from every light it can see behind it. - Accellerometers and gyrosopes on the robot can keep track of its orientation. - Using the video from the camera and knowledge of its orientation, the robot can always determine it's XYZ position relative to the farthest light it can see. - By tracking its motion relative to the lights it leaves behind, it can track it's absolute XYZ position in the pipe system.
The advantage of this system is that the robot will often be able to see a light that is pretty far away, and make an absolute reading of its position relative to that. This reduces the accumulation of small measurement errors that you might get with other systems that only track motion incrementally.
-- Matt
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Interesting. How about this:
- Put closely-spaced lights on the cable that flash in sequence. - The robot keeps the cable tight as the lights unwind. Perhaps it needs to install spring-loaded braces every now and then to accomplish this. - A video camera on the robot can observe the lights it leaves behind. - By tracking the amount of unwound cable, the robot can determine its distance from every light it can see behind it. - Accellerometers and gyrosopes on the robot can keep track of its orientation. - Using the video from the camera and knowledge of its orientation, the robot can always determine it's XYZ position relative to the farthest light it can see. - By tracking its motion relative to the lights it leaves behind, it can track it's absolute XYZ position in the pipe system.
The advantage of this system is that the robot will often be able to see a light that is pretty far away, and make an absolute reading of its position relative to that. This reduces the accumulation of small measurement errors that you might get with other systems that only track motion incrementally.
-- Matt
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On Tue, 04 Oct 2005 14:12:31 GMT, Frnak McKenney

I don't know of any silly approaches but there are two I think would work ok. One is to have the robot send out a radio pulse every second or so and build that into a map. The directional aerial could be a bit difficult to make. The next approach is to have an ultrasonic rangefinding with a tight beam emitter suitable for pipes. Everytime the robot makes a turn, ping the opposite wall and build the figures into a map. Some sort of angle detector might be needed to handle curves, both horizontal and vertical.
Sig: "The cat was once revered as a god. They have never forgotten this" - Anon
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