Any designs for a cheap x-y position sensor?

I think that George guy who was asking about radio front ends a few weeks ago was probably doing something like that. Three transmitters and a bit of signal processing will give you three intersecting hyperbolas for a given set of phase differences.

Acoustics has much shorter wavelengths, but is more vulnerable to wind and multiple reflections.

Another approach would be mechanical triangulation: two stakes, some very thin steel cable, two spring-loaded reels, and two rotary encoders. Stretch one piece of cable between the stakes, so you know the baseline, and connect the other ends of the cables to a third stake. You get the two radii from the encoders, and can triangulate to your position.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
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hobbs at electrooptical dot net
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Laser scanner, I believe that is what they use at building sites these day

Laser and rotating mirror measuring angles and maybe distance to reference points I assume

-Lasse

Maybe you can extend this:

Near electromagnetic field strength measurement could return coordinates in

100 x 100 ft square with better then 1 ft accuracy. Put a source of a field at unknown location, and two receivers at known locations; solve for the position. It could be done vise versa: two transmitters at known location and a receiver at unknown location. Of course, this method is prone to EMI and distortions due to large conductive or ferrous objects; however it is simple and it could be more robust then acoustics, optics or EM trilateration.

Vladimir Vassilevsky DSP and Mixed Signal Consultant

In a controlled environment (evidently not applicable) large scale (billboard+) printers have been built using a spark gap (for acoustic generation) on the print head and microphones at the edges. Read about it, don't know the details, I'd assume the controller makes a spark and only counts the first impulse at each microphone. IIRC the use of a spark was in part to get a nice "sharp" sound that was not easily confused with background.

Some variant on laser rangefinding would do the job, but cheap and 100ms might not coincide too well. Park some retroreflective prisms at the corners and go to town (look at what surveyors use these days - a "total station" - good to 1/100th of a foot anyway at moderate ranges.) Depending on available optics, moving the prism around and parking the station would also work. For actual available tech, two bodies, and much longer than 100mS updates (well, you have to walk the prism and park it to get a reading - I don't know how fast the TS can track it as it moves), one guy to point the TS at the prism and one guy to walk the prism around the field is a typical approach, at least until the TS needs to move in order to see the prism. That also gets you 3D.

I don't have a TS to play with, but TS and GPS are the reason I have a transit that "used to be" reasonably nice and is now (and has been for decades) "obsolete."

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A bunch of cheap USB cameras, looking down on the scene from various angles, and a bunch of software.

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John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
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Precision electronic instrumentation
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Optical version of a Aircraft VOR. Diode lasers are cheap. Mount them at the corners and sides of the field on a rotating stage. Fan out the beam with a cylindrical lens. You transmit a sync pulse, either optically or by other means, when the laser is at true north. Look for the laser light using a bandpass filter. Find the leading edge of the beam as it passes over a detector. This gives you the angle at a known time. Any two of them gives a fix. There are plenty of papers on the net on multilateration.

If you have the leading and trailing edges of the beam, and its diverging, you also have a measure of the distance in the far field of the source.

Cheaper then time of flight.

Steve

100 feet resolved to 0.25 inch is 4800 points. In X and Y that's a 4800 x 4800 grid. A 23 Mpixel camera would do it. Or four carefully overlapped 8Mpixel cameras.

I don't know how flat a field of view they give you. Stitching four fields together, and keeping the cameras sufficiently rigidly aligned to keep the stitching stable, might well be challenging.

--
Bill Sloman, Nijmegen

Scanning lasers are only used for level (altitude) AFAIK

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Subpixel resoulution should be possible, so it may be doable with lower resolution cameras.

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On a sunny day (Mon, 17 Sep 2012 14:11:42 -0700 (PDT)) it happened Robert Macy wrote in :

Pole with webcam looking down at you? The rest is software... :-)

Two surveyed wooden posts along a ~100' baseline and an ultrasonic tape measure. You will have to put a repeater on top of the posts to send back the pulse since in clear air there will not be enough echo.

Realistically you would probably want something to do the maths for you.

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Martin Brown

Acoustics is interesting, but I've got the soundcard tied up. Didn't think of it, but could back off and go after 8 channel 44100 rates.

Interesting approach, a 'mechanical' solution. I can picture the 'tether' method. Like two tape measures attached to two poles. Accuracy when directly between the two poles is the pits, but best around 45 degrees from each. The 'field' easily extends past the two poles also. Sadly, being in a remote area precludes lugging a lot of weight up into the region. Plus, being tethered seems fraught with physical breakage problems. But still, a somewhat unusual approach, which might have applications for another project - inspection sites, where the whole thing could be a fixture.

I think you might be right to just use a robust surveying tool. Doesn't Zircon out of Campbell, CA make a laser tape measure for cheap?

Wow! didn't think of that at all! Thought of a mouse-like rolling thingy, but the approach in that URL is much more self contained and actually fits our existing software!

We have a prize winner!

in

Great approach. Firt thing I thought of, too. but alas, I have some fields already in existence around the system that the introduction of any new field is likely to cause problems.

Just a footnote to readers: Distance measuring using magnetic fields is really cheap and fairly accurate [relative terms]. I designed a portable distance measuring system for a Medical Product which essentially measured displacement along 16 axes, simultaneously. We built a simple two axis unit for measuring displacement from 6 inches to over 27 inches. Cost less than $5. No coil winding. Power consumption less than 5 mA per axis at 5Vdc. The exciting thing was that the MEASURED noise at a displacement of 8 inches was a mere 1 mil rms!!

ay.

Acoustics, I haven't considered very thoroughly, but could have some effectiveness.Will think about that one.

The prism idea is interesting for its accuracy. But I think Ineed this to be 'single' operator system.

Hadn't considered optics much. But there is a lot of power in using 'many' cheap units and sorting things out in SW

Wow! didn't think of that at all! Thought of a mouse-like rolling thingy, but the approach in that URL is much more self contained and actually fits our existing software!

We have a prize winner!

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Good luck getting it to work over rough ground.