making a bot follow an invisible track

I want to make vehicles crawl around a LEGO town following a path I define -- but in some invisible way, so the onlookers can't see how it
works. What are my options? Here are some I've thought of, but perhaps you can think of something better:
1. Put strong magnets under the road, every few cm, maybe closer together on the curves. Magnetic sensors of some sort (maybe just reed switches would do?) in the car would use this to steer, much like a traditional optical line-follower. Problems: the road can be almost a cm thick itself, which would necessitate the use of strong (and therefore expensive) magnets.
2. Put cheap iron bars or balls (or whatever) under the road, and a strong magnet on a swivel in the car. The magnet should point at the nearest iron, and we can detect that (maybe by bouncing an LED off it) and use it to steer. I'm not sure if this would work reliably.
3. Er... getting short on ideas here, but if we could embed some sort of navigation beacons in the town layout, then in principle you could make the cars "programmable" by manually pushing them once around the path you want them to follow, and they should be able to repeat that path again and again. But I'm not sure what sort of navigation beacons would be appropriate -- remember, neither the car nor the town is supposed to have any visible gadgetry; it's supposed to look like the minifigs are just driving their cars.
4. Really scraping the barrel, could we do some sort of optical line-follower but not in the visible light range? Something we could use to paint a line on the road that would be invisible to humans, but visible to the car's sensors?
Any insight on the above ideas, or new ideas I haven't considered?
Many thanks, - Joe
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A buried wire fed by an ac signal for the bot to center on would probably be a good way to go.
Mark
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Can you elaborate on this? What sort of sensors would the bot use to detect such a wire?
Thanks, - Joe
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Thanks, that looks very useful!
Best, - Joe
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I think option 4. would be cheapest. You could use one of those secret message pen/marker sets that are in dollar stores to draw your line. Then use a UV(Ultra violet lamp/led under your car to light up your line for you line tracker. I'm not an expert but it might just work.

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I like this idea. Basically, you use a moving "black room" over your sensors (a good shade) to keep out outside light, and illuminate the interior with UV LED's. The rest is straight line following. But the best sensors would be CdS, because of their broad sensitivity sepctrum. Dpa uses these and a white light source on SR04 quite successfully. He uses white light to illuminate the line, and can then follow white lines on black, or black on white, etc.
OTOH, most of the line sensors, like the QRB1134 are IR based. It is easy to use UV to cause flourescence by down conversion of higher frequency light to lower level band gap drops. I don't think it is nearly as easy to get up-shifting of IR into the visible (a process of double absorbtion of lower energy photons with final release of a higher one). However, if you were to stay in the IR band, there might be a substance that would be IR reflective, yet transparent to visible light, and then, you could line follow an IR reflective line with just existing line follow equipment.
-- Randy M. Dumse www.newmicros.com Objects in mirror are more confused than they appear.
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That does seem like a good idea. Something like this maybe: <(Amazon.com product link shortened) />/ B000GATXIY/ref=pd_bxgy_k_text_b/002-4586816-9189602> ...which is under $3 for the marker. Then I'd just need a UV LED, maybe something like Mouser part 889-LU531, which is also under $3.
Even better than a permanent ink would be one that can be washed off... LEGO fans hate to permanently modify their parts, even with something invisible. But these inks are usually used for security labeling, and they wouldn't be much good for that if they were washable. So I might have to just live with permanent ink.

Yes. I haven't heard of any such IR inks though. And a regular LEGO surface (even a black one, I think) reflects IR pretty well, so we'd have trouble achieving much contrast that way.
Thanks for the ideas, guys! - Joe
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Sorry to bump an old post, but I just ran across this and remembered this thread.
Invisible IR Ink: http://www.maxmax.com/aXRayIRInks.asp
Sounds handy. IR detection would probably be much easier and cheaper than UV.
-Robotguy http://robotguy.net/blog
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Well, the idea with the UV ink is to simple shine a UV LED on it (these are pretty cheap), and then detect the visible-light fluorescence.
But that would mean that anybody peeking under the car could see a glowing line. This IR ink should be completely invisible even while the bot is tracking it. Sounds very neat -- thank you!
Best, - Joe
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Robotguy wrote:

Several years ago I experimented with infrared inks as a camcorder pirate deterrent. The idea was to "spraypaint" (using a robotic rig similar to those in painting signs) an identifying number or mark over the screen in a theater. Xenon light sources, common in theatrical projectors, carry strong lines in the near infrared region of 800-1000nm. Most camcorders, even color and those with infrared blocking filters on them, are still infrared sensitive. The ink was of the type that fluoresced only into the infrared region, so the human eye could not see it. The idea was that trying to video the screen would result in these ID numbers appearing throughout the movie, making for a lousy pirated tape.
The problem was one of cost as these inks and applying them are pretty expensive -- even before getting the theaters interested, and they don't like to spend money on anything. Then I had the idea of doing it with a small IR laser from the projection booth, but it wasn't bright enough. Back to the drawing board...
-- Gordon
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wrote:

The light source doesn't need to be a laser. How about the light source being a set of IR LED's, which are arranged in a pattern(perhaps representing a binary number), then using a regular lens to focus their light onto the screen. Alternatively, the LEDs can be behind the screen. Perhaps they would shine through the screen material, or small holes can be made for the light to come through.
Joe Dunfee
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Joe wrote:

Already covered by a patent from another company (Macrovision). Simple "spoiling" IR lights are already used in some theaters.

Well-known suggestion in the industry, and generally too expensive to implement (it *is* in use in some theaters). The actual hardware isn't too expensive, at least now, but the installation labor is a couple thousand per screen, more than what most theaters are willing to pay to protect someone else's IP. When you consider providing power (gotta be OSHA/UL approved), creating the scaffolding/harness to hang the devices (they can't just be glued to the back of the screen), the signal wiring from the booth, the cost of the IR clusters, etc., it really adds up.
Thing is, with digital cinema rolling out, and the ability to insert individualized forensic marking in real time, the IR solutions are taking a second seat.
-- Gordon
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